cnct: make original htlc amt available for contract resolvers

Previously, contract resolvers that needed to publish a second level tx,
did not have access to the original htlc amount.

This commit reconstructs this amount from data that is already persisted
in arbitrator log.

Co-authored-by: Joost Jager <joost.jager@gmail.com>

contractcourt/htlc_timeout_resolver.go

@@ -40,6 +40,10 @@ type htlcTimeoutResolver struct {
 	// additional commitment state machine.
 	htlcIndex uint64
 
+	// htlcAmt is the original amount of the htlc, not taking into
+	// account any fees that may have to be paid if it goes on chain.
+	htlcAmt lnwire.MilliSatoshi
+
 	ResolverKit
 }