Introduce cAPI.UnitState() for a single unit

[dongsupark]

At the moment cAPI doesn't support any possibility for retrieving unit states of a single unit. Instead fleet needs to call cAPI.UnitStates() to fetch states for all units, which is of course not efficient.

To support cAPI.UnitState() for a single unit, we need to do the following:

• Add UnitState() to the API interface in client/api.go as well as to the HTTPClient in client/http.go.
• Add a struct UnitStateGetCall and its necessary methods like Get(), Fields(), Do() in schema/v1-gen.go. Of course schema/v1-json.go needs to be regenerated.
• Add UnitState() to the UnitRegistry interface in registry/interface.go
• Add UnitState() also to both EtcdRegistry and RPCRegistry.
• Add GetUnitState() to the Registry service in protobuf/fleet.proto. Of course protobuf/fleet.pb.go needs to be regenerated.

Not sure if I've missed anything.

See also #1667 (comment)