lnwire: add specification 'AnnounceSignature' message

In this commit announcement signature message has been added which is
needed when peers want to announce their channel to the rest of the
network. This message acts as half proof carrier, nodes exchanges
their half proofs with each other and after that they are able to
construct the full proof.

lnwire/message.go

@@ -55,6 +55,7 @@ const (
 	CmdChannelAnnoucmentMessage       = uint32(5000)
 	CmdChannelUpdateAnnoucmentMessage = uint32(5010)
 	CmdNodeAnnoucmentMessage          = uint32(5020)
+	CmdAnnounceSignatures             = uint32(5030)
 
 	// Commands for connection keep-alive.
 	CmdPing = uint32(6000)
@@ -128,6 +129,8 @@ func makeEmptyMessage(command uint32) (Message, error) {
 		msg = &NodeAnnouncement{}
 	case CmdPing:
 		msg = &Ping{}
+	case CmdAnnounceSignatures:
+		msg = &AnnounceSignatures{}
 	case CmdPong:
 		msg = &Pong{}
 	default: