Add new hack annotations for thrift compiler

Summary:
As discussed in thrift language and tooling meeting, let's use annotations instead of compiler options for union migration.

These annotations currently don't do anything. I just want to get the new annotations synced to other repos so we can begin codemods.

Reviewed By: rmakheja

Differential Revision: D79417779

fbshipit-source-id: 30cc993402890c2e8b15c96641cb47459b89ac65

Author: Dan Deng

thrift/annotation/hack.thrift

@@ -210,3 +210,21 @@ struct ModuleInternal {}
 @scope.Service
 @scope.Function
 struct GenerateClientMethodsWithHeaders {}
+
+/**
+ * Generated Hack types are being changed to `final` by default.
+ *
+ * For existing thrift types that are inherited from and cannot be easily migrated,
+ * use this annotation to make the generated class non-final.
+ */
+@scope.Structured
+struct MigrationBlockingAllowInheritance {}
+
+/**
+ * Hack Unions without the `protected` compiler option may rely on `fb_json_serialize` for JSON serialization.
+ * Adding this annotation will implement the JSONSerializable Hack interface allowing `fb_json_serialize` to continue being used.
+ *
+ * Note: This should only be used for migration purposes. Use `JSONThriftSerializer` instead.
+ */
+@scope.Union
+struct MigrationBlockingLegacyJSONSerialization {}