Add optional attribute to spec

The current Recap null-field handling is a bit cumbersome. I discussed with
@gwukelic, @adrianisk, and @cpard. We decided to add support for an `optional`
field to the CST. This will allow us to specify that a field is optional.

See the conversation here:

gabledata/recap#335

Users can now specify an optional field as:

```yaml
type: struct
fields:
  - name: secondary_phone
    type: string32
    optional: true
```

This will get treated the same as:

```yaml
type: struct
fields:
  - name: secondary_phone
    type: "union",
    types: ["null", "string32"],
    default: null
```

specs/type/0.2.0.md

@@ -209,16 +209,11 @@ An ordered collection of Recap types. Table schemas are typically represented as
 #### `field` Attributes
 {: .no_toc }
 
-Recap types in the `fields` attribute can have two extra attributes set: `name` and `default`.
+Recap types in the `fields` attribute can have an extra `name` attribute to set a field's name.
 
 | Name | Description | Type | Required | Default |
 |------|-------------|------|----------|---------|
 | name | The field's name. | String | NO | |
-| default | The default value for a reader if the field is not set in the struct. | Literal of any type | NO | |
-
-An unset default is differentiated from a default with a null value. An unset default is treated as "no default", while a default that's been set to null is treated as a null default.
-
-*NOTE: Database defaults often appear as strings like `nextval('\"public\".some_id_seq'::regclass)` or `'USD'::character varying`. Such defaults are left to the developer to interpret based on the database they're using.*
 
 #### Examples
 {: .no_toc }
@@ -235,18 +230,6 @@ fields:
     bytes: 255
 ```
 
-Optional fields are expressed as a union with a `null` type and a null default (similar to [Avro's fields](https://avro.apache.org/docs/1.10.2/spec.html#schema_record)).
-
-```yaml
-# A struct with an optional string field called "secondary_phone"
-type: struct
-fields:
-  - name: secondary_phone
-    type: union
-    types: ["null", "string32"]
-    default: null
-```
-
 ### `enum`
 
 An enumeration of string symbols.
@@ -314,6 +297,93 @@ types:
       bits: 32
 ```
 
+## Defaults
+
+Recap types can have a `default` attribute with a literal value. The value is used when the field is not set in a struct.
+
+| Name | Description | Type | Required | Default |
+|------|-------------|------|----------|---------|
+| default | The default value for a reader if the field is not set in the struct. | Literal of any type | NO | |
+
+```yaml
+# A struct with a field called "id" that defaults to 0
+type: struct
+fields:
+  - name: id
+    type: int
+    bits: 32
+    default: 0
+```
+
+A missing default is differentiated from a default with a null value. An unset default is treated as "no default", while a default that's been set to null is treated as a null default.
+
+*NOTE: Database defaults often appear as strings like `nextval('\"public\".some_id_seq'::regclass)` or `'USD'::character varying`. Such defaults are left to the developer to interpret based on the database they're using.*
+
+## Optional Types
+
+Optional types are expressed as a union with a `null` type and a null default (similar to [Avro's fields](https://avro.apache.org/docs/1.10.2/spec.html#schema_record)).
+
+```yaml
+# A struct with an optional string field called "secondary_phone"
+type: struct
+fields:
+  - name: secondary_phone
+    type: union
+    types: ["null", "string32"]
+    default: null
+```
+
+Unions can be cumbersome, so Recap supports a shorthand syntax for types using `optional`:
+
+```yaml
+# A struct with an optional string field called "secondary_phone"
+type: struct
+fields:
+  - name: secondary_phone
+    type: string32?
+    optional: true
+```
+
+The above shorthand is equivalent to the union example.
+
+Optional types also work for unions:
+
+```yaml
+# A union type of null or a 32-bit signed int
+type: union
+types:
+  - type: int32
+  - type: float32
+optional: true
+```
+
+Optionals also work with aliases:
+
+```yaml
+# A struct with an optional string field called "secondary_phone"
+type: struct
+fields:
+  - name: phone
+    alias: phone_type
+    type: string32
+  - name: secondary_phone
+    type: phone_type
+    optional: true
+```
+
+Optionality is not inherited in aliases:
+
+```yaml
+type: struct
+fields:
+  - name: phone
+    alias: phone_type
+    type: string32
+    optional: true
+  - name: secondary_phone
+    type: phone_type
+```
+
 ## Logical Types
 
 Logical types annotate one of the 11 Recap types listed in the *Types* section at the top of the spec. Logical types add additional context to Recap types that help converters. For example, a `decimal` logical type can be defined as:
@@ -665,4 +735,13 @@ Recap comes with a collection of built-in aliases:
 * `time64`: Time since midnight without timezones and leap seconds in a 64-bit signed integer.
 * `timestamp64`: Time elapsed (in a time unit) since a specific epoch.
 * `date32`: Date since the UNIX epoch without timezones and leap seconds in a 32-bit integer.
-* `date64`: Date since the UNIX epoch without timezones and leap seconds in a 64-bit integer.
+* `date64`: Date since the UNIX epoch without timezones and leap seconds in a 64-bit integer.
+
+## Changelog
+
+* 0.2.0
+  * Add `optional` attribute. ([#335](https://github.com/recap-build/recap/pull/335))
+* 0.1.1
+  * Make `string` and `bytes` byte attribute optional. ([#284](https://github.com/recap-build/recap/discussions/284) [#292](https://github.com/recap-build/recap/pull/292) [#293](https://github.com/recap-build/recap/pull/293))
+* 0.1.0
+  * Initial release.