htlcswitch: add new ProcessContractResolution method

In this commit, we add a new method: ProcessContractResolution. This
will be used by entities of the contract court package to notify us
whenever they discover that we can resolve an incoming contract
off-chain after the outgoing contract was fully resolved on-chain.

We’ll take a contractcourt.ResolutionMsg and map it to the proper
internal package so we can fully resolve an active circuit.

htlcswitch/packet.go

@@ -46,4 +46,11 @@ type htlcPacket struct {
 	// of a forwarded fail packet are already set and do not need to be looked
 	// up in the circuit map.
 	isRouted bool
+
+	// isResolution is set to true if this packet was actually an incoming
+	// resolution message from an outside sub-system. We'll treat these as
+	// if they emanated directly from the switch. As a result, we'll
+	// encrypt all errors related to this packet as if we were the first
+	// hop.
+	isResolution bool
 }