Include service specific status in query response

[pks-t]

When clients query a service, they might want to know about e.g. the service's current status, availability, configuration or capabilities. E.g. for a display service, the service description may contain the display resolution.

A previous approach was to have a set of parameters with optional default values which were returned. This approach was not really sufficient in that it was very constrained. A new approach might involve the service plugin filling out a service-specific protobuf message, which then contains all infomation on the service.