rpc: Add PendingOpenChannel to SubscribeChannelEvents

This commit adds PendingOpenChannel to SubscribeChannelEvents stream in
the gRPC API.
This is useful for keeping track of channel openings that Autopilot does.
It can also be used for the non-initator side of a channel opening to keep
track of channel openings.

lntest/itest/lnd_test.go

@@ -6460,25 +6460,34 @@ func testBasicChannelCreationAndUpdates(net *lntest.NetworkHarness, t *harnessTe
 	// Since each of the channels just became open, Bob and Alice should
 	// each receive an open and an active notification for each channel.
 	var numChannelUpds int
-	const totalNtfns = 2 * numChannels
+	const totalNtfns = 3 * numChannels
 	verifyOpenUpdatesReceived := func(sub channelSubscription) error {
 		numChannelUpds = 0
 		for numChannelUpds < totalNtfns {
 			select {
 			case update := <-sub.updateChan:
 				switch update.Type {
-				case lnrpc.ChannelEventUpdate_ACTIVE_CHANNEL:
-					if numChannelUpds%2 != 1 {
-						return fmt.Errorf("expected open" +
-							"channel ntfn, got active " +
+				case lnrpc.ChannelEventUpdate_PENDING_OPEN_CHANNEL:
+					if numChannelUpds%3 != 0 {
+						return fmt.Errorf("expected " +
+							"open or active" +
+							"channel ntfn, got pending open " +
 							"channel ntfn instead")
 					}
 				case lnrpc.ChannelEventUpdate_OPEN_CHANNEL:
-					if numChannelUpds%2 != 0 {
-						return fmt.Errorf("expected active" +
+					if numChannelUpds%3 != 1 {
+						return fmt.Errorf("expected " +
+							"pending open or active" +
 							"channel ntfn, got open" +
 							"channel ntfn instead")
 					}
+				case lnrpc.ChannelEventUpdate_ACTIVE_CHANNEL:
+					if numChannelUpds%3 != 2 {
+						return fmt.Errorf("expected " +
+							"pending open or open" +
+							"channel ntfn, got active " +
+							"channel ntfn instead")
+					}
 				default:
 					return fmt.Errorf("update type mismatch: "+
 						"expected open or active channel "+