Work on interop_client

[kolmodin]

There's a description of interop tests. Not all tests are implemented;

• [x] empty_unary
• [ ] cacheable_unary
• [x] large_unary
• [ ] client_compressed_unary
• [ ] server_compressed_unary
• [x] client_streaming
• [ ] client_compressed_streaming
• [x] server_streaming
• [ ] server_compressed_streaming
• [x] ping_pong
• [x] empty_stream
• [ ] compute_engine_creds
• [ ] jwt_token_creds
• [ ] oauth2_auth_token
• [ ] per_rpc_creds
• [x] custom_metadata
• [x] status_code_and_message
• [x] unimplemented_method
• [x] unimplemented_service
• [x] cancel_after_begin
• [ ] cancel_after_first_response
• [x] timeout_on_sleeping_server
• [ ] concurrent_large_unary