grpcio: improve server interceptor typing

[RobinMcCorkell]

ServerInterceptor and GenericRpcHandler should not be generic over request/response themselves, but rather should have generic methods:

• ServerInterceptor: 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
• GenericRpcHandler (and ServiceRpcHandler): this is "An implementation of arbitrarily many RPC methods" and has a service method that takes a parameter describing the method to handle, but that parameter dictates the request/response types. The instance of the GenericRpcHandler itself does not know about the request/response types. In theory this could be described using dependent types, but since this is Python the best we can do is have the service method return a type with Anys and allow the caller to cast later

Related: #14641

[github-actions]

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

[RobinMcCorkell]

@srittau as you reviewed the previous grpcio PR, could you review this one please?

[srittau]

Thanks, good catch!