Add @thrift.Deprecated annotation definition

Summary:
`thrift.Deprecated` is a structured annotation that can be applied to various Thrift definitions, to mark them as deprecated in their target language. This diff initially focusses on field access, to mark certain fields as "don't use anymore", in the migration period before they can be deleted.

When applied, code generators can emit language-specific deprecation markers (e.g., `[[deprecated]]` in C++, `Deprecated` in Java) that produce compiler warnings when deprecated elements are used. The annotation includes an optional `message` field to explain why the element is deprecated and what to use instead, e.g:

```thrift
struct User {
  thrift.Deprecated{message = "Use 'full_name' instead"}
  1: string name;
  2: string full_name;
}
```

## Devnote

The fixture tests are empty, so each subsequent commit indicates what the changes are for each language.

Reviewed By: praihan

Differential Revision: D92099551

fbshipit-source-id: fcbdbe6fcd82c8551aa8edc25df8a8c489c9a6cb

Author: sadroeck

thrift/annotation/thrift.thrift

@@ -429,3 +429,25 @@ struct AllowLegacyMissingUris {}
 @scope.Typedef
 @scope.FunctionParameter
 struct AllowUnsafeNonSealedKeyType {}
+
+/**
+ * Marks a definition as deprecated.
+ *
+ * When applied, generated code will include language-specific deprecation
+ * markers that produce compiler warnings when the deprecated element is used.
+ *
+ * Example:
+ *   struct User {
+ *     @thrift.Deprecated{message = "Use 'full_name' instead"}
+ *     1: string name;
+ *     2: string full_name;
+ *   }
+ */
+@scope.Field
+struct Deprecated {
+  /**
+   * Explanation of why this is deprecated and what to use instead.
+   * This message will appear in compiler warnings.
+   */
+  1: string message;
+}