node: Make node_supports_v3_rendezvous_point() also check for the onion_pk
Turns out that we think this is related to legacy/trac#27774 (moved).
A client selects a v3 RP by only looking at the protover but it is possible that at that time we simply don't have the descriptor yet thus missing the
onion_pk which led to a failure when sending the INTRODUCE1 cell.
node_supports_ed25519_link_authentication() does, we should simply check for the
onion_pk as an extra step to the protover.
Flagging this for backport.