Construct and sign `SingleAttestation` in the validators attestation service

[eserilev]

The validator attestation service constructs an Attestation object that it signs, converts to a SingleAttestation and publishes to the network. We could remove the conversion step by just constructing the SingleAttestation directly and signing it. There is no need to support the pre-Electra unaggregated attestation format.

[PoulavBhowmick03]

Does this mean we should add a empty_for_signing method in SingleAttestation and call it directly instead of creating the Attestation object? and also add a function in the ValidatorStore trait to sign single attestations?