multi: obtain+verify aux sigs for all second level HTLCs

In this commit, we start to use the new AuxSigner to obtain+verify aux sigs for all second level HTLCs. This is similar to the existing SigPool, but we'll only attempt to do this if the AuxSigner is present (won't be for most channels).

config_builder.go

@@ -174,6 +174,10 @@ type AuxComponents struct {
 	// able to automatically handle new custom protocol messages related to
 	// the funding process.
 	AuxFundingController fn.Option[funding.AuxFundingController]
+
+	// AuxSigner is an optional signer that can be used to sign auxiliary
+	// leaves for certain custom channel types.
+	AuxSigner fn.Option[lnwallet.AuxSigner]
 }
 
 // DefaultWalletImpl is the default implementation of our normal, btcwallet
@@ -580,6 +584,7 @@ func (d *DefaultWalletImpl) BuildWalletConfig(ctx context.Context,
 		ChanStateDB:                 dbs.ChanStateDB.ChannelStateDB(),
 		NeutrinoCS:                  neutrinoCS,
 		AuxLeafStore:                aux.AuxLeafStore,
+		AuxSigner:                   aux.AuxSigner,
 		ActiveNetParams:             d.cfg.ActiveNetParams,
 		FeeURL:                      d.cfg.FeeURL,
 		Fee: &lncfg.Fee{
@@ -737,6 +742,7 @@ func (d *DefaultWalletImpl) BuildChainControl(
 		NetParams:             *walletConfig.NetParams,
 		CoinSelectionStrategy: walletConfig.CoinSelectionStrategy,
 		AuxLeafStore:          partialChainControl.Cfg.AuxLeafStore,
+		AuxSigner:             partialChainControl.Cfg.AuxSigner,
 	}
 
 	// The broadcast is already always active for neutrino nodes, so we
@@ -919,10 +925,6 @@ type DatabaseInstances struct {
 	// for native SQL queries for tables that already support it. This may
 	// be nil if the use-native-sql flag was not set.
 	NativeSQLStore *sqldb.BaseDB
-
-	// AuxLeafStore is an optional data source that can be used by custom
-	// channels to fetch+store various data.
-	AuxLeafStore fn.Option[lnwallet.AuxLeafStore]
 }
 
 // DefaultDatabaseBuilder is a type that builds the default database backends