Collator Protocol Revamp: emit new ApprovedPeer event on candidate inclusion

[alindima]

Add a new event emitted by the relay chain runtime whenever a candidate is included, which contains the approved peer sent by the parachain over UMP. This will be used by collator protocol for bumping reputations.

It makes sense to emit this on inclusion rather than backing, in order to make sure that backing an invalid candidate and then not making it available would result in a slash. Otherwise, backers could use this to bump the system-wide reputations of select collators without doing the work of creating valid blocks.

An alternative would be to explicitly query the candidates_pending_availability at the previous relay parent, but this would be more wasteful and would use an extra runtime API which potentially returns a lot of extra data.

[alindima]

maybe it's a good idea to find a different name for the event, because it's the same as the ump signal name

[sandreim]

I still don't see a good reason to make this an event. We can just have a runtime API that returns all the PeerIds from these included candidates.

[alindima]

I just think it's simpler. You process much less data and you don't need to go back to a block where the candidates were still pending availability.
Anyway, I think either approach is fine really, and it's not a high priority issue.

We can just rely on the candidates_pending_availability runtime API for a while