Support async Authorizers

[Zsailer]

Add support for asynchronous Authorizers.

This is feature additive—it shouldn't affect standard, synchronous authorizers. This enables folks to make the is_authorized method asynchronous which is particularly useful for fetching authorization status from an external auth provider.

[blink1073]

 tests/auth/test_authorizer.py:241: in AsyncAuthorizerTest
    ) -> Awaitable | bool:
E   TypeError: unsupported operand type(s) for |: '_GenericAlias' and 'type'

Probably need a from __future__ import annotations in that file.

[Zsailer]

I think I fixed it by being more explicit about what the Awaitable returns 😃