Optimise `SingleAttestation` implementation further

[michaelsproul]

For v7.0.0 we took the a straight-forward approach for SingleAttestations, translating them into Attestations and keeping the core of Lighthouse's processing the same.

It would be more efficient to reverse the direction of this translation so that we can benefit from the structure of SingleAttestations during verification, i.e. so we can check signatures prior to hitting committee caches.

We've begun on this, and plan to release it shortly after Electra goes live. More info and ideas contained in the WIP implementations:

• Fuller implementation of SingleAttestation #6923 (newer)
• SingleAttestation full implementation #6616 (older)