lnwire: validate that gossip messages contain valid TLV

In this commit, we check that the extra bytes appended to gossip
messages contain valid TLV streams. We do this here for:
- channel_announcement
- channel_announcement_2
- channel_update
- channel_update_2
- node_announcement

This is in preparation for the SQL version of the graph store which will
normalise TLV streams at persistence time.

docs/release-notes/release-notes-0.20.0.md

@@ -34,7 +34,8 @@
 * Graph Store SQL implementation and migration project:
   * Introduce an [abstract graph 
     store](https://github.com/lightningnetwork/lnd/pull/9791) interface. 
- 
+  * Start [validating](https://github.com/lightningnetwork/lnd/pull/9787) that 
+    byte blobs at the end of gossip messages are valid TLV streams.
 
 ## RPC Updates