contractcourt: add aux chan closer to chain watcher

Due to a circular dependency issue, we cannot directly import these
definitions from lnwallet/chancloser. So we'll redefine the types we
need and define a slim interface around the aux closer, which contains
exactly the functionality that we'll want to use here.

Author: GeorgeTsagk

contractcourt/chain_arbitrator.go

@@ -230,6 +230,10 @@ type ChainArbitratorConfig struct {
 	// AuxResolver is an optional interface that can be used to modify the
 	// way contracts are resolved.
 	AuxResolver fn.Option[lnwallet.AuxContractResolver]
+
+	// AuxCloser is an optional interface that can be used to finalize
+	// cooperative channel closes.
+	AuxCloser fn.Option[AuxChanCloser]
 }
 
 // ChainArbitrator is a sub-system that oversees the on-chain resolution of all
@@ -1138,6 +1142,7 @@ func (c *ChainArbitrator) WatchNewChannel(newChan *channeldb.OpenChannel) error
 			extractStateNumHint: lnwallet.GetStateNumHint,
 			auxLeafStore:        c.cfg.AuxLeafStore,
 			auxResolver:         c.cfg.AuxResolver,
+			auxCloser:           c.cfg.AuxCloser,
 		},
 	)
 	if err != nil {
@@ -1315,6 +1320,7 @@ func (c *ChainArbitrator) loadOpenChannels() error {
 				extractStateNumHint: lnwallet.GetStateNumHint,
 				auxLeafStore:        c.cfg.AuxLeafStore,
 				auxResolver:         c.cfg.AuxResolver,
+				auxCloser:           c.cfg.AuxCloser,
 			},
 		)
 		if err != nil {

contractcourt/chain_watcher.go

@@ -24,6 +24,7 @@ import (
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnutils"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/types"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -37,6 +38,14 @@ const (
 	maxCommitPointPollTimeout = 10 * time.Minute
 )
 
+// AuxChanCloser is used to allow an external caller to finalize a cooperative
+// channel close.
+type AuxChanCloser interface {
+	// FinalizeClose is called after the close transaction has been agreed
+	// upon and confirmed.
+	FinalizeClose(desc types.AuxCloseDesc, closeTx *wire.MsgTx) error
+}
+
 // LocalUnilateralCloseInfo encapsulates all the information we need to act on
 // a local force close that gets confirmed.
 type LocalUnilateralCloseInfo struct {
@@ -229,6 +238,9 @@ type chainWatcherConfig struct {
 
 	// auxResolver is used to supplement contract resolution.
 	auxResolver fn.Option[lnwallet.AuxContractResolver]
+
+	// auxCloser is used to finalize cooperative closes.
+	auxCloser fn.Option[AuxChanCloser]
 }
 
 // chainWatcher is a system that's assigned to every active channel. The duty