multi: add new config option upfront-shutdown-address

Introduced a new config value `upfront-shutdown-address`
in the `lnd.conf` file. This ensures that channel close
funds are transferred to the specified shutdown address.
The value applies to both the funder and the fundee but
can be overridden by the value specified during
`openchannel` or by the `channel acceptor`.

NOTE: If this field is set when opening a channel with a
peer that does not advertise support for upfront shutdown
feature, the channel open will fail.

Signed-off-by: Nishant Bansal <nishant.bansal.282003@gmail.com>

funding/manager.go

@@ -573,6 +573,10 @@ type Config struct {
 	// implementations to inject and process custom records over channel
 	// related wire messages.
 	AuxChannelNegotiator fn.Option[lnwallet.AuxChannelNegotiator]
+
+	// ShutdownScript is an optional upfront-shutdown script to which our
+	// funds should be paid on a cooperative close.
+	ShutdownScript fn.Option[lnwire.DeliveryAddress]
 }
 
 // Manager acts as an orchestrator/bridge between the wallet's
@@ -1760,12 +1764,24 @@ func (f *Manager) fundeeProcessOpenChannel(peer lnpeer.Peer,
 		return
 	}
 
+	// If the fundee didn't provide an upfront-shutdown address via
+	// the channel acceptor, fall back to the configured shutdown
+	// script (if any).
+	shutdownScript := acceptorResp.UpfrontShutdown
+	if len(shutdownScript) == 0 {
+		f.cfg.ShutdownScript.WhenSome(
+			func(script lnwire.DeliveryAddress) {
+				shutdownScript = script
+			},
+		)
+	}
+
 	// Check whether the peer supports upfront shutdown, and get a new
 	// wallet address if our node is configured to set shutdown addresses by
 	// default. We use the upfront shutdown script provided by our channel
 	// acceptor (if any) in lieu of user input.
 	shutdown, err := getUpfrontShutdownScript(
-		f.cfg.EnableUpfrontShutdown, peer, acceptorResp.UpfrontShutdown,
+		f.cfg.EnableUpfrontShutdown, peer, shutdownScript,
 		f.selectShutdownScript,
 	)
 	if err != nil {
@@ -4849,12 +4865,23 @@ func (f *Manager) handleInitFundingMsg(msg *InitFundingMsg) {
 		}
 	}
 
+	// If the funder did not provide an upfront-shutdown address, fall back
+	// to the configured shutdown script (if any).
+	shutdownScript := msg.ShutdownScript
+	if len(shutdownScript) == 0 {
+		f.cfg.ShutdownScript.WhenSome(
+			func(script lnwire.DeliveryAddress) {
+				shutdownScript = script
+			},
+		)
+	}
+
 	// Check whether the peer supports upfront shutdown, and get an address
 	// which should be used (either a user specified address or a new
 	// address from the wallet if our node is configured to set shutdown
 	// address by default).
 	shutdown, err := getUpfrontShutdownScript(
-		f.cfg.EnableUpfrontShutdown, msg.Peer, msg.ShutdownScript,
+		f.cfg.EnableUpfrontShutdown, msg.Peer, shutdownScript,
 		f.selectShutdownScript,
 	)
 	if err != nil {