This change adds support for `@api_stub` for the base resource class, used for "special" links (create/update/delete/etc) as well as for any resource-specific links. This also adds a new decorator, `@request_method_returns`, which works like `@request_method`, but can plumb through a specific return type. This has been used to make `ResourceLinkField` generic, so that calls to a `ResourceLinkField.get()` method can specify their return type as a specific resource subclass. Stubs for common special methods have been added to Resource, ItemResource, and ListResource. This change also adds resource-specific stubs to ReviewRequestItemResource, for those links where we have existing resource classes to reference. I've also added type hints for the review request attributes. Testing Done: - Ran unit tests. - Smoke tested rbt commands. - Ran a test script with debug logging to see that the stub debug info looked as expected.