Port mock-service to use kuksa-client lib

[SebastianSchildt]

Currently Mock is not very maintainable as it uses its homespun GRPC wrappers. This is a lot of code dupblication, instead it should use the well/better tested kuksa-client lib

Potential issue: It might rely on pending "discoverability" issue in the val.v1 API. In any case, I suggest we start moving as mich calls as possible over to the API now, and follow up with the rest once upstream updated kuksa-client and databroker API

[SebastianSchildt]

Done in #88