Add support to loopback RPCs

[felippeduran]

The current implementation doesn't support RPC calls to the same server or server type. Although it's nice to block this functionality to avoid developer error, there are some scenarios where it could be interesting to have those. One example is to isolate features in a monolithic server to then migrate later to separate services more easily.

One downside of there RPCs is the unnecessary overhead. In order to avoid such problem, this merge request creates a loopback flow inside the RemoteService class, bypassing nats or gRPC implementations and calling the remote handlers directly. In order to keep an intuitive trace, it also mimics the span structure of regular RPCs.

Tests have been fixed/edited to incorporate the new use case.