Python: grpcio-testing support for server interceptors

### Is your feature request related to a problem? Please describe.
As far as I can tell, unit testing a gRPC server with its interceptors is not possible.

**This leads to 2 issues :**
- It is not possible to unit test servers which **require** that a specific interceptor exists (e.g. transforming the input query)
- [Possibly wrong] It is not possible to unit test the interceptors themselves

The root cause seems to be that it is not possible to pass an `interceptors` param to a test gRPC server (e.g. `grpc_testing.server_from_dictionary`) - in the same way we can with "true" gRPC Python servers (e.g. `grpc.server`).

### Describe the solution you'd like
Be able to pass an `interceptors` params to a test gRPC server.

### Describe alternatives you've considered
- Replace interceptors with decorators, at least for unit tests -> leads to code complexity
- Not use grpcio-testing and call methods implementing RPCs directly -> Less test coverage (no gRPC level testing)