Attempt to remove `SingleAttestation` conversion and deal with it in the `AggregatingAttestationPool`

[tbenr]

The current SingleAttestation conversion is a bit unclean and lead us to some stupid bugs (in publishing and in the performance calculator)

We could try to avoid that and implement single attestation handling in AggregatingAttestationPool itself (maybe actually MatchingDataAttestationGroup).
This will probably boils down to make AttestationBitsAggregator compatible to that, which will require missing bits calculated via getSingleAttestationAggregationBits to be stored in the ValidatableAttestation.

Don't know if this will end up being a better solution but could worth a try.

Note: we have to make sure we always produce an AttestationElectra when we have to produce an aggregation (thinking about the edge case where we have only one SingleAttestation in the pool)