Consolidate ValidationQueue and PendingAttestations

[HCastano]

In #1063 there were some discussions about potentially streamlining the data structures used to
manage attestations.

We manage attestations in two places right now:

1. In the Staking Extension pallet, through the use of ValidationQueue
2. In the Attestation pallet, through the use of PendingAttestations and AttestationRequests

We update the ValidationQueue in the Attestation pallet through the use of some
associated types, so it could make sense for us to abstract all that logic out in a
similar fashion.

It would be great it this could let us get rid of those two storage structures as well as
the Attestation pallet's on_initialize hook which is updating them based off the
entried in ValidationQueue.