EIP-7549 redesign - SingleAttestation and OnchainAttestation

[mehdi-aouadi]

Refactor the EIP-7549 implementation to separate the unaggregated attestation from aggregated ones. Spec PRs:

1. Separate type for onchain attestation aggregates
2. Separate type for unaggregated network attestations

Schema definitions changes:

• [ ] Rollback Attestation containers changes (remove phase0 and electra attestations)
• [ ] Introduce SingleAttestation container
• [ ] Introduce OnchainAttestation container
• [ ] Update the BeaconBlockBody to use the new OnchainAttestation container
• [ ] Update the IndexedAttestation container to use the new OnchainAttestation container
• [ ] Update the AttesterSlashing container to use the new IndexedAttestation

Attestation processing changes:

• [ ] Add the new compute_signing_attestation_data
• [ ] Add the new get_onchain_attesting_indices
• [ ] Implement the new process_onchain_attestation which replaces the existing process_attestation

Gossip changes:

• [ ] Update the beacon_attestation_{subnet_id} topic to handle the new SingleAttestation
• [ ] Update the attestation pool to handle SingleAttestation and OnchainAttestation

Beacon API changes:

• [ ] Rollback the GET /eth/v2/validator/aggregate_attestation API
• [ ] Update the GET /eth/v2/beacon/blocks/{block_id}/attestations API
• [ ] Update the GET /eth/v2/beacon/pool/attestations API
• [ ] Update the POST /eth/v2/beacon/pool/attestations API
• [ ] Update the GET /eth/v2/beacon/pool/attester_slashings API
• [ ] Update the POST /eth/v2/beacon/pool/attester_slashings API