Dedicated API for testing interceptors

[v0idpwn]

Is your feature request related to a problem? Please describe.
Currently, there's no way to test server interceptors without defining an endpoint and servers. Applications willing to test interceptors in isolation must define this boilerplate.

Describe the solution you'd like
Something like Plug.Test, where I can get a stream and a request and apply interceptors.

Describe alternatives you've considered
Stay without dedicated testing API, but document how one can test servers/interceptors and maybe reduce the necessary boilerplate (e.g.: if the library included a test service, you wouldn't need to define a test service in your application)

[polvalente]

Perhaps a GRPC.Testing module can be provided, and there might be a way for it to help define said testing modules more easily.

+1 for exploring this route (no pun intended)

[sleipnir]

@v0idpwn @polvalente Can we explore this further? What would we expect to inject into the test module?