prop224: Add an introduction point onion key in the descriptor

So it turns out that this menagerie of keys in prop224 was missing yet one more key in it :D.

To extend to an introduction point, we need its onion key meaning we have to add it to the descriptor. Keep in mind that the client does NOT lookup the node in its consensus in order not to reveal which consensus it's using and avoid reachability issues with different consensus between service and client.

This goes in two fold. First add an onion key field in the spec and then implement it in an extra commit in #20657 (moved) code (which is being reviewed). I would like us to avoid making a branch depending on 8k lines of code from another unmerged ticket.