Uniter Action(id string) API client/server

This commit adds the ability for Uniter clients to query Actions from
state.State by id.  The response includes a server error message if the
Action is not present on the state server.
We're not trying to access the unit. We're trying to figure out whether the entity embodied by the Auth is allowed to see that *action* -- that'll be determined by (1) if it's an action for that unit, yes it can and potentially later (2) if the unit's currently the service leader, and it's an action for the service, yes it can. Otherwise errperm.