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).

peer/brontide.go

@@ -376,6 +376,10 @@ type Config struct {
 	// leaves for certain custom channel types.
 	AuxLeafStore fn.Option[lnwallet.AuxLeafStore]
 
+	// AuxSigner is an optional signer that can be used to sign auxiliary
+	// leaves for certain custom channel types.
+	AuxSigner fn.Option[lnwallet.AuxSigner]
+
 	// PongBuf is a slice we'll reuse instead of allocating memory on the
 	// heap. Since only reads will occur and no writes, there is no need
 	// for any synchronization primitives. As a result, it's safe to share
@@ -952,6 +956,9 @@ func (p *Brontide) loadActiveChannels(chans []*channeldb.OpenChannel) (
 		p.cfg.AuxLeafStore.WhenSome(func(s lnwallet.AuxLeafStore) {
 			chanOpts = append(chanOpts, lnwallet.WithLeafStore(s))
 		})
+		p.cfg.AuxSigner.WhenSome(func(s lnwallet.AuxSigner) {
+			chanOpts = append(chanOpts, lnwallet.WithAuxSigner(s))
+		})
 		lnChan, err := lnwallet.NewLightningChannel(
 			p.cfg.Signer, dbChan, p.cfg.SigPool, chanOpts...,
 		)
@@ -4164,6 +4171,9 @@ func (p *Brontide) addActiveChannel(c *lnpeer.NewChannel) error {
 	p.cfg.AuxLeafStore.WhenSome(func(s lnwallet.AuxLeafStore) {
 		chanOpts = append(chanOpts, lnwallet.WithLeafStore(s))
 	})
+	p.cfg.AuxSigner.WhenSome(func(s lnwallet.AuxSigner) {
+		chanOpts = append(chanOpts, lnwallet.WithAuxSigner(s))
+	})
 
 	// If not already active, we'll add this channel to the set of active
 	// channels, so we can look it up later easily according to its channel

peer/test_utils.go

@@ -304,6 +304,8 @@ func createTestPeerWithChannel(t *testing.T, updateChan func(a,
 	alicePool := lnwallet.NewSigPool(1, aliceSigner)
 	channelAlice, err := lnwallet.NewLightningChannel(
 		aliceSigner, aliceChannelState, alicePool,
+		lnwallet.WithLeafStore(&lnwallet.MockAuxLeafStore{}),
+		lnwallet.WithAuxSigner(&lnwallet.MockAuxSigner{}),
 	)
 	if err != nil {
 		return nil, err
@@ -316,6 +318,8 @@ func createTestPeerWithChannel(t *testing.T, updateChan func(a,
 	bobPool := lnwallet.NewSigPool(1, bobSigner)
 	channelBob, err := lnwallet.NewLightningChannel(
 		bobSigner, bobChannelState, bobPool,
+		lnwallet.WithLeafStore(&lnwallet.MockAuxLeafStore{}),
+		lnwallet.WithAuxSigner(&lnwallet.MockAuxSigner{}),
 	)
 	if err != nil {
 		return nil, err