node: Make node_supports_v3_rendezvous_point() also check for the onion_pk

Turns out that we think this is related to #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.

Like 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.