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.

channelnotifier/channelnotifier.go

@@ -20,6 +20,13 @@ type ChannelNotifier struct {
 	chanDB *channeldb.DB
 }
 
+// PendingOpenChannelEvent represents a new event where a new channel has
+// entered a pending open state.
+type PendingOpenChannelEvent struct {
+	// ChannelPoint is the channelpoint for the new channel.
+	ChannelPoint *wire.OutPoint
+}
+
 // OpenChannelEvent represents a new event where a channel goes from pending
 // open to open.
 type OpenChannelEvent struct {
@@ -82,6 +89,16 @@ func (c *ChannelNotifier) SubscribeChannelEvents() (*subscribe.Client, error) {
 	return c.ntfnServer.Subscribe()
 }
 
+// NotifyPendingOpenChannelEvent notifies the channelEventNotifier goroutine that a
+// new channel is pending.
+func (c *ChannelNotifier) NotifyPendingOpenChannelEvent(chanPoint wire.OutPoint) {
+	event := PendingOpenChannelEvent{ChannelPoint: &chanPoint}
+
+	if err := c.ntfnServer.SendUpdate(event); err != nil {
+		log.Warnf("Unable to send pending open channel update: %v", err)
+	}
+}
+
 // NotifyOpenChannelEvent notifies the channelEventNotifier goroutine that a
 // channel has gone from pending open to open.
 func (c *ChannelNotifier) NotifyOpenChannelEvent(chanPoint wire.OutPoint) {