Minimal single attestation

[tbenr]

This spike seems to work now.

It implements the minimal impact strategy, where we perform SingleAttestation<->Attestation conversion to minimize changes.

In particular:

• AttestationSubnetSubscriptions uses SingleAttestation schema when creates the electra single attestation topic handler
• in ValidatableAttestation:
  • attestaiton become modifiable (to cover the conversion transparently)
  • optionally holds SingleAttestation version of the attestation (to be used when gossiping)
• during attestation validation, if we are dealing with a SingleAttestation, we:
  • change the validation so that we verify signature before checking committee (which triggers the shuffling calculation)
  • convert SingleAttestation attestation to Attestation and cache the original (happening inside ValidatableAttestation)
  • keep creating IndexedAttestation from SingleAttestation but I doubt it is useful
• during attestation validation, if we are dealing with a non aggregated Attestation produced locally, we:
  • convert Attestation attestation to SingleAttestation (happening inside ValidatableAttestation)
• in AttestationGossipManager, when publishing unaggregated attestation we get it via validatableAttestation.getSingleAttestation() which:
  • pre-electra will give back the phase0 standard Attestation
  • from electra will give back SingleAttestation as result of the conversion happened during validation.

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.