lnwire: add Outpoint to ChannelAnnouncement2

The latest version of the spec has the outpoint included in the
`channel_announcement2` message.

lnwire/channel_announcement_2_test.go

@@ -58,6 +58,17 @@ func TestChanAnn2EncodeDecode(t *testing.T) {
 		0xb3, 0xee, 0x17, 0x2f, 0x7f, 0x16, 0x1, 0xe6, 0x7d, 0x1d, 0xa6,
 		0xca, 0xd4, 0xb, 0x54, 0xc4, 0x46, 0x8d, 0x48, 0x23, 0x6c, 0x39,
 
+		// Outpoint record.
+		0x12, // type (18).
+		0x22, // length (34 bytes).
+		// Hash (32 bytes).
+		0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
+		0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10,
+		0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18,
+		0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x20,
+		// Index (2 bytes).
+		0x30, 0x39, // value: 12345.
+
 		// Unknown TLV record.
 		0x6f,       // type.
 		0x2,        // length.