multi: update WalletController PublishTransaction to include label

Add label parameter to PublishTransaction in WalletController
interface. A labels package is added to store generic labels that are
used for the different types of transactions that are published by lnd.

To keep commit size down, the two endpoints that require a label
parameter be passed down have a todo added, which will be removed in
subsequent commits.

watchtower/lookout/justice_descriptor_test.go

@@ -293,7 +293,7 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 	// over the buffered channel.
 	publications := make(chan *wire.MsgTx, 1)
 	punisher := lookout.NewBreachPunisher(&lookout.PunisherConfig{
-		PublishTx: func(tx *wire.MsgTx) error {
+		PublishTx: func(tx *wire.MsgTx, _ string) error {
 			publications <- tx
 			return nil
 		},

watchtower/lookout/punisher.go

@@ -8,7 +8,7 @@ import (
 type PunisherConfig struct {
 	// PublishTx provides the ability to send a signed transaction to the
 	// network.
-	PublishTx func(*wire.MsgTx) error
+	PublishTx func(*wire.MsgTx, string) error
 
 	// TODO(conner) add DB tracking and spend ntfn registration to see if
 	// ours confirmed or not
@@ -42,7 +42,7 @@ func (p *BreachPunisher) Punish(desc *JusticeDescriptor, quit <-chan struct{}) e
 	log.Infof("Publishing justice transaction for client=%s with txid=%s",
 		desc.SessionInfo.ID, justiceTxn.TxHash())
 
-	err = p.cfg.PublishTx(justiceTxn)
+	err = p.cfg.PublishTx(justiceTxn, "")
 	if err != nil {
 		log.Errorf("Unable to publish justice txn for client=%s"+
 			"with breach-txid=%s: %v",