lnwallet/chancloser: create unique ServiceKey for the RBF chan closer

This can be used to allow any system to send a message to the RBF chan
closer if it knows the proper service key. In the future, we can use
this to redo the msgmux.Router in terms of the new actor abstractions.

Author: Roasbeef

lnwallet/chancloser/rbf_coop_states.go

@@ -1026,3 +1026,7 @@ type RbfEvent = protofsm.EmittedEvent[ProtocolEvent]
 // RbfStateSub is a type alias for the state subscription type of the RBF chan
 // closer.
 type RbfStateSub = protofsm.StateSubscriber[ProtocolEvent, *Environment]
+
+// ChanCloserActorMsg is an adapter to enable the state machine executor that
+// runs this state machine to be passed around as an actor.
+type ChanCloserActorMsg = protofsm.ActorMessage[ProtocolEvent]