BOLT 7: add gossip address descriptor type DNS hostname

This introduces a new gossip address descriptor type used for DNS hostnames.
This is particular useful for dynamic DNS users that want to use their home
ISP connection with changing IP addresses without relying only on TOR.

The `len` field is deliberately encoded with just a byte (u8) since
POSIX hostnames do not exceed 255 bytes in total.

07-routing-gossip.md

@@ -285,6 +285,7 @@ The following `address descriptor` types are defined:
          onion service addresses; Encodes:
          `[32:32_byte_ed25519_pubkey] || [2:checksum] || [1:version]`, where
          `checksum = sha3(".onion checksum" | pubkey || version)[:2]`.
+   * `5`: DNS hostname; data = `[byte:len][len*byte:hostname][u16:port]` (length up to 258)
 
 ### Requirements