Update the beacon block body

[mehdi-aouadi]

Block proposal

Constructing the BeaconBlockBody

Attestations

The network attestation aggregates contain only the assigned committee attestations.
Attestation aggregates received by the block proposer from the committee aggregators with disjoint committee_bits sets and equal AttestationData SHOULD be consolidated into a single Attestation object.
The proposer should run the following function to construct an on chain final aggregate form a list of network aggregates with equal AttestationData:

def compute_on_chain_aggregate(network_aggregates: List[Attestation]) -> Attestation:
    aggregates = sorted(network_aggregates, key=lambda a: get_committee_indices(a.committee_bits)[0])

    data = aggregates[0].data
    aggregation_bits = Bitlist[MAX_VALIDATORS_PER_COMMITTEE * MAX_COMMITTEES_PER_SLOT]()
    for a in aggregates:
        for b in a.aggregation_bits:
            aggregation_bits.append(b)

    signature = bls.Aggregate([a.signature for a in aggregates])

    committee_indices = [get_committee_indices(a.committee_bits)[0] for a in aggregates]
    committee_flags = [(index in committee_indices) for index in range(0, MAX_COMMITTEES_PER_SLOT)]        
    committee_bits = Bitvector[MAX_COMMITTEES_PER_SLOT](committee_flags)

    return Attestation(aggregation_bits, data, committee_bits, signature)