grpcio: improve client interceptors

[RobinMcCorkell]

Follow up to #14785

ClientInterceptors should not be generic over request/response themselves, but rather should have generic methods: a given instance of this should be able to intercept any call, and so should have a generic method over all request/continuation types rather than the instance being specialized itself.

I also fixed the continuation/return types on the client interceptors which were incorrect:

• The type of the second parameter to the continuation should be identical to the request/iterator given to the intercept call
• The return type of the continuation should be identical to the return type of the intercept call
• aio interceptor continuations are always Awaitable

The new tests demonstrate the correct usage.

Fixes: #14641

[RobinMcCorkell]

@srittau one more for you :)

[srittau]

A note on the tests: We don't run the tests, we only run type checkers against them, which is fairly expensive. Which is why we only use tests for weird corner cases and especially complex type annotations.

While the previous annotations were wrong, the new annotations aren't particular complex, so I don't think we need tests. The comments before the abstract methods should make the intent of the generics clear.

[RobinMcCorkell]

Fair point, I removed the new tests and expanded the comments to clarify the invariant that the types need to satisfy.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Thanks!