doc.protocol: Add gRPC documentation

[romainletendart]

The main intent of this PR is to discuss about error handling with the new unified gRPC interface.
This also makes a good opportunity to start documenting (or at least listing) all the gRPC interfaces available.

[SebastianSchildt]

@romainletendart @argerus Not sure, what the status is here currently, the text still lists "current state" and "desired state" for error handling. Wasn't the plan to move over to "desired" implementation wise, and just document that here?

[romainletendart]

@SebastianSchildt I would not document something as "implemented" if the implementation is not there yet.
I would instead mark this PR as blocked by #385.
Once we have implemented the new error handling, we update this PR accordingly I would say.

[lukasmittag]

Does this need a merge or further work?

[erikbosch]

@SebastianSchildt - I think we need to have a general discussion/decision on how we want error handling to look like in the long term, and as well what changes we want to do in the short time.

For now I think the discussion and work-in-progress fits better in a Wiki-page so i created https://github.com/eclipse/kuksa.val/wiki/KUKSA.val-gRPC-interfaces. I suggest this PR can be closed, when we have agreed on the way forward we can create a new PR