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