[PROTOCOL RFC] Variant data type (delta-io#2867)

#### Which Delta project/connector is this regarding?

- [x] Spark
- [ ] Standalone
- [ ] Flink
- [ ] Kernel
- [ ] Other (fill in here)

## Description

Adds the protocol changes for the Variant data type (see
delta-io#2864) to the RFC folder.

## How was this patch tested?

N/A

## Does this PR introduce _any_ user-facing changes?

N/A

protocol_rfcs/README.md

@@ -22,6 +22,7 @@ Here is the history of all the RFCs propose/accepted/rejected since Feb 6, 2024,
 | 2023-02-09    | [type-widening.md](https://github.com/delta-io/delta/blob/master/protocol_rfcs/type-widening.md)                                  | https://github.com/delta-io/delta/issues/2623 | Type Widening                 |
 | 2023-02-14    | [managed-commits.md](https://github.com/delta-io/delta/blob/master/protocol_rfcs/managed-commits.md)                              | https://github.com/delta-io/delta/issues/2598 | Managed Commits               |
 | 2023-02-26    | [column-mapping-usage.tracking.md](https://github.com/delta-io/delta/blob/master/protocol_rfcs/column-mapping-usage-tracking.md)) | https://github.com/delta-io/delta/issues/2682 | Column Mapping Usage Tracking |
+| 2023-04-24    | [variant-type.md](https://github.com/delta-io/delta/blob/master/protocol_rfcs/variant-type.md)                                    | https://github.com/delta-io/delta/issues/2864 | Variant Data Type             |
 
 ### Accepted RFCs
 

protocol_rfcs/variant-type.md

@@ -0,0 +1,99 @@
+# Variant Data Type
+**Associated Github issue for discussions: https://github.com/delta-io/delta/issues/2864**
+
+This protocol change adds support for the Variant data type.
+The Variant data type is beneficial for storing and processing semi-structured data.
+
+--------
+
+> ***New Section after the [Clustered Table](#clustered-table) section***
+
+# Variant Data Type
+
+This feature enables support for the Variant data type, for storing semi-structured data.
+The schema serialization method is described in [Schema Serialization Format](#schema-serialization-format).
+
+To support this feature:
+- The table must be on Reader Version 3 and Writer Version 7
+- The feature `variantType` must exist in the table `protocol`'s `readerFeatures` and `writerFeatures`.
+
+## Example JSON-Encoded Delta Table Schema with Variant types
+
+```
+{
+  "type" : "struct",
+  "fields" : [ {
+    "name" : "raw_data",
+    "type" : "variant",
+    "nullable" : true,
+    "metadata" : { }
+  }, {
+    "name" : "variant_array",
+    "type" : {
+      "type" : "array",
+      "elementType" : {
+        "type" : "variant"
+      },
+      "containsNull" : false
+    },
+    "nullable" : false,
+    "metadata" : { }
+  } ]
+}
+```
+
+## Variant data in Parquet
+
+The Variant data type is represented as two binary encoded values, according to the [Spark Variant binary encoding specification](https://github.com/apache/spark/blob/master/common/variant/README.md).
+The two binary values are named `value` and `metadata`.
+
+When writing Variant data to parquet files, the Variant data is written as a single Parquet struct, with the following fields:
+
+Struct field name | Parquet primitive type | Description
+-|-|-
+value | binary | The binary-encoded Variant value, as described in [Variant binary encoding](https://github.com/apache/spark/blob/master/common/variant/README.md)
+metadata | binary | The binary-encoded Variant metadata, as described in [Variant binary encoding](https://github.com/apache/spark/blob/master/common/variant/README.md)
+
+The parquet struct must include the two struct fields `value` and `metadata`.
+Supported writers must write the two binary fields, and supported readers must read the two binary fields.
+Struct fields which start with `_` (underscore) can be safely ignored.
+
+## Writer Requirements for Variant Data Type
+
+When Variant type is supported (`writerFeatures` field of a table's `protocol` action contains `variantType`), writers:
+- must write a column of type `variant` to parquet as a struct containing the fields `value` and `metadata` and storing values that conform to the [Variant binary encoding specification](https://github.com/apache/spark/blob/master/common/variant/README.md)
+- must not write additional, non-ignorable parquet struct fields. Writing additional struct fields with names starting with `_` (underscore) is allowed.
+
+## Reader Requirements for Variant Data Type
+
+When Variant type is supported (`readerFeatures` field of a table's `protocol` action contains `variantType`), readers:
+- must recognize and tolerate a `variant` data type in a Delta schema
+- must use the correct physical schema (struct-of-binary, with fields `value` and `metadata`) when reading a Variant data type from file
+- must make the column available to the engine:
+    - [Recommended] Expose and interpret the struct-of-binary as a single Variant field in accordance with the [Spark Variant binary encoding specification](https://github.com/apache/spark/blob/master/common/variant/README.md).
+    - [Alternate] Expose the raw physical struct-of-binary, e.g. if the engine does not support Variant.
+    - [Alternate] Convert the struct-of-binary to a string, and expose the string representation, e.g. if the engine does not support Variant.
+
+## Compatibility with other Delta Features
+
+Feature | Support for Variant Data Type
+-|-
+Partition Columns | **Supported:** A Variant column is allowed to be a non-partitioned column of a partitioned table. <br/> **Unsupported:** Variant is not a comparable data type, so it cannot be included in a partition column.
+Clustered Tables | **Supported:** A Variant column is allowed to be a non-clustering column of a clustered table. <br/> **Unsupported:** Variant is not a comparable data type, so it cannot be included in a clustering column.
+Delta Column Statistics | **Supported:** A Variant column supports the `nullCount` statistic. <br/> **Unsupported:** Variant is not a comparable data type, so a Variant column does not support the `minValues` and `maxValues` statistics.
+Generated Columns | **Supported:** A Variant column is allowed to be used as a source in a generated column expression, as long as the Variant type is not the result type of the generated column expression. <br/> **Unsupported:** The Variant data type is not allowed to be the result type of a generated column expression.
+Delta CHECK Constraints | **Supported:** A Variant column is allowed to be used for a CHECK constraint expression.
+Default Column Values | **Supported:** A Variant column is allowed to have a default column value.
+Change Data Feed | **Supported:** A table using the Variant data type is allowed to enable the Delta Change Data Feed.
+
+--------
+
+> ***New Sub-Section after the [Map Type](#map-type) sub-section within the [Schema Serialization Format](#schema-serialization-format) section***
+
+### Variant Type
+
+Variant data uses the Delta type name `variant` for Delta schema serialization.
+
+Field Name | Description
+-|-
+type | Always the string "variant"