Clients correlated with Service definitions for server/client streaming services

[juanpedromoreno]

We should cover services with these variants:

• Clients correlated with Service definitions for client streaming services
• Clients correlated with Service definitions for server streaming services
• Clients correlated with Service definitions for bidirectional streaming services

Part of this job will be to figure out which pattern fits better for observer/observable patterns.

[juanpedromoreno]

After some investigation, we can provide Observable as the data type by default to manage the streaming APIs, avoiding the use of the Java StreamObserver type.

As you can see in the Design Summary section, in terms of frees algebras, assuming that R is the output type for each service:

• unary services and client-to-server streaming services: they can simply return FS[R].
• server-to-client and bi-directional services: they would return FS[monix.reactive.Observable[R]].