Handle authentication at servicer level

Context

Authentication, as introduced for #144 (closed), is implemented using a gRPC interceptor. The Python API for interceptors is limited in that it does not give access to the RPC context at interception time, thus making it difficult to pass authorization information down to the RPC handling code.

A solution to this could be to stop using an interceptor but to handle authentication at servicer level (ie. at an explicit check to every gRPC exposed method).

Task Description

• Move authentication handling down to servicers.