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 :)

[github-actions]

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