feat(spanner): add proto and enum types

PiperOrigin-RevId: 599046867

google/spanner/v1/type.proto

@@ -47,6 +47,13 @@ message Type {
   // typically is not needed to process the content of a value (it doesn't
   // affect serialization) and clients can ignore it on the read path.
   TypeAnnotationCode type_annotation = 4;
+
+  // If [code][google.spanner.v1.Type.code] ==
+  // [PROTO][google.spanner.v1.TypeCode.PROTO] or
+  // [code][google.spanner.v1.Type.code] ==
+  // [ENUM][google.spanner.v1.TypeCode.ENUM], then `proto_type_fqn` is the fully
+  // qualified name of the proto type representing the proto/enum definition.
+  string proto_type_fqn = 5;
 }
 
 // `StructType` defines the fields of a [STRUCT][google.spanner.v1.TypeCode.STRUCT] type.
@@ -145,6 +152,13 @@ enum TypeCode {
   //   preserved.
   // - JSON array elements will have their order preserved.
   JSON = 11;
+
+  // Encoded as a base64-encoded `string`, as described in RFC 4648,
+  // section 4.
+  PROTO = 13;
+
+  // Encoded as `string`, in decimal format.
+  ENUM = 14;
 }
 
 // `TypeAnnotationCode` is used as a part of [Type][google.spanner.v1.Type] to