Collator Protocol Revamp: add new ApprovedPeer UMP signal

[alindima]

Add a new ApprovedPeer variant to the UMPSignal enum, which will be used to promote the reputation of a collator by the parachain runtime.
It must be optional.
Implement helper functions for querying it (probably on the CommittedCandidateReceipt).

Relax the assumption that we only have one UMP signal (which was the core selector). Allow at most 2 ump signals, one core selector and one approved peer (we could have none, either one of them or both).

This needs to be coordinated through a node feature, because we must make sure that all validators are aware of this new signal. Otherwise, we could create disputes and potential slashes to otherwise innocent validators. Before the node feature is enabled, getting an ApprovedPeer ump signal needs to remain an error.