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.

Ran unit tests.
Smoke tested rbt commands.
Ran a test script with debug logging to see that the stub debug info

  looked as expected.