Adapt NextMessageHop serialization to 0.2.3 fork ser macros

The upstream lightningdevkit#4682 commits use impl_ser_tlv_based_enum!, which does not
exist on the 0.2.3_fork base. Use the fork's equivalent
impl_writeable_tlv_based_enum!, which supports the same tuple-variant
syntax and produces a self-consistent Readable/Writeable impl.

Amp-Thread-ID: https://ampcode.com/threads/T-019f1bf1-a8e4-745c-8492-3d1c9ff270a0
Co-authored-by: Amp <amp@ampcode.com>

Author: Synesso

lightning/src/blinded_path/message.rs

@@ -258,7 +258,7 @@ pub enum NextMessageHop {
 	ShortChannelId(u64),
 }
 
-impl_ser_tlv_based_enum!(NextMessageHop,
+impl_writeable_tlv_based_enum!(NextMessageHop,
 	{0, NodeId} => (),
 	{2, ShortChannelId} => (),
 );