lnwire: add new ChanUpdateFlag for the ChannelUpdate flag bitmask

In this commit, we add a new type to the lnwire package:
ChanUpdateFlag. This type represent the bitfield that’s used within the
ChannelUpdate message to give additional details as how the message
should be interpreted.

lnwire/lnwire_test.go

@@ -427,7 +427,7 @@ func TestLightningWireProtocol(t *testing.T) {
 				Signature:       testSig,
 				ShortChannelID:  NewShortChanIDFromInt(uint64(r.Int63())),
 				Timestamp:       uint32(r.Int31()),
-				Flags:           uint16(r.Int31()),
+				Flags:           ChanUpdateFlag(r.Int31()),
 				TimeLockDelta:   uint16(r.Int31()),
 				HtlcMinimumMsat: MilliSatoshi(r.Int63()),
 				BaseFee:         uint32(r.Int31()),