lnwire: add AnnounceSignatures interface

Add a AnnounceSignatures interface and ensure that AnnounceSignatures1
implements it.

lnwire/interfaces.go

@@ -0,0 +1,13 @@
+package lnwire
+
+// AnnounceSignatures is an interface that represents a message used to
+// exchange signatures of a ChannelAnnouncment message during the funding flow.
+type AnnounceSignatures interface {
+	// SCID returns the ShortChannelID of the channel.
+	SCID() ShortChannelID
+
+	// ChanID returns the ChannelID identifying the channel.
+	ChanID() ChannelID
+
+	Message
+}