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).

funding/manager.go

@@ -554,6 +554,10 @@ type Config struct {
 	// able to automatically handle new custom protocol messages related to
 	// the funding process.
 	AuxFundingController fn.Option[AuxFundingController]
+
+	// AuxSigner is an optional signer that can be used to sign auxiliary
+	// leaves for certain custom channel types.
+	AuxSigner fn.Option[lnwallet.AuxSigner]
 }
 
 // Manager acts as an orchestrator/bridge between the wallet's
@@ -1083,6 +1087,9 @@ func (f *Manager) advanceFundingState(channel *channeldb.OpenChannel,
 	f.cfg.AuxLeafStore.WhenSome(func(s lnwallet.AuxLeafStore) {
 		chanOpts = append(chanOpts, lnwallet.WithLeafStore(s))
 	})
+	f.cfg.AuxSigner.WhenSome(func(s lnwallet.AuxSigner) {
+		chanOpts = append(chanOpts, lnwallet.WithAuxSigner(s))
+	})
 
 	// We create the state-machine object which wraps the database state.
 	lnChannel, err := lnwallet.NewLightningChannel(

funding/manager_test.go

@@ -567,6 +567,9 @@ func createTestFundingManager(t *testing.T, privKey *btcec.PrivateKey,
 		AuxLeafStore: fn.Some[lnwallet.AuxLeafStore](
 			&lnwallet.MockAuxLeafStore{},
 		),
+		AuxSigner: fn.Some[lnwallet.AuxSigner](
+			&lnwallet.MockAuxSigner{},
+		),
 	}
 
 	for _, op := range options {
@@ -677,6 +680,7 @@ func recreateAliceFundingManager(t *testing.T, alice *testNode) {
 		DeleteAliasEdge:       oldCfg.DeleteAliasEdge,
 		AliasManager:          oldCfg.AliasManager,
 		AuxLeafStore:          oldCfg.AuxLeafStore,
+		AuxSigner:             oldCfg.AuxSigner,
 	})
 	require.NoError(t, err, "failed recreating aliceFundingManager")