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.

rpcserver.go

@@ -3187,6 +3187,16 @@ func (r *rpcServer) SubscribeChannelEvents(req *lnrpc.ChannelEventSubscription,
 		case e := <-channelEventSub.Updates():
 			var update *lnrpc.ChannelEventUpdate
 			switch event := e.(type) {
+			case channelnotifier.PendingOpenChannelEvent:
+				update = &lnrpc.ChannelEventUpdate{
+					Type: lnrpc.ChannelEventUpdate_PENDING_OPEN_CHANNEL,
+					Channel: &lnrpc.ChannelEventUpdate_PendingOpenChannel{
+						PendingOpenChannel: &lnrpc.PendingUpdate{
+							Txid:        event.ChannelPoint.Hash[:],
+							OutputIndex: event.ChannelPoint.Index,
+						},
+					},
+				}
 			case channelnotifier.OpenChannelEvent:
 				channel, err := createRPCOpenChannel(r, graph,
 					event.Channel, true)
@@ -5601,7 +5611,9 @@ func (r *rpcServer) SubscribeChannelBackups(req *lnrpc.ChannelBackupSubscription
 			switch e.(type) {
 
 			// We only care about new/closed channels, so we'll
-			// skip any events for active/inactive channels.
+			// skip any events for pending/active/inactive channels.
+			case channelnotifier.PendingOpenChannelEvent:
+				continue
 			case channelnotifier.ActiveChannelEvent:
 				continue
 			case channelnotifier.InactiveChannelEvent: