Update attestation construction

[mehdi-aouadi]

Attesting

Construct attestation

• Set attestation_data.index = 0.
• Let aggregation_bits be a Bitlist[MAX_VALIDATORS_PER_COMMITTEE] of length len(committee), where the bit of the index of the validator in the committee is set to 0b1.
• Set attestation.aggregation_bits = [aggregation_bits], a list of length 1
• Let committee_bits be a Bitvector[MAX_COMMITTEES_PER_SLOT], where the bit at the index associated with the validator's committee is set to 0b1
• Set attestation.committee_bits = committee_bits

Note: Calling get_attesting_indices(state, attestation) should return a list of length equal to 1, containing validator_index.