Examples for L2 API

[evshary]

While #122 is almost complete, would we also be able to provide examples of how to use these APIs?
I'm trying to do similar things in up-transport-zenoh-rust, but I found something unclear to me while doing the implementation.
For example, I quite struggled to understand the meaning of RequestHandler in InMemoryRpcServer and how to use it elegantly.

[sophokles73]

I see, there is no unit test using a real implementation of RequestHandler. However, test_request_listener_invokes_operation_successfully() should give you a good impression of what the RequestHandler is supposed to do. The only thing missing in the test case is the registration of the endpoint via RpcServer::register_endpoint ...

[sophokles73]

In general I would also like to have examples. For that we would need a (simple) UTransport implementation which might even simply dispatch messages to listeners in-memory only.