Update Attestation APIs with SingleAttestation

[mehdi-aouadi]

Update the Attestation APIs to handle the new SingleAttestation
Spec PR: ethereum/beacon-APIs#472

[tbenr]

It is not sure we will go in that direction. Depends by how much we want to minimize the impacts when introducing SingleAttestation.

[mehdi-aouadi]

It is not sure we will go in that direction. Depends by how much we want to minimize the impacts when introducing SingleAttestation.

Yes, agree. Just logged this issue to keep track and not forget about it in case it gets merged

[rolfyone]

i believe this is done now

[tbenr]

fixed via #8893 and #8873