Add getAggregatedAttestationV2 to support EIP-7549

[ensi321]

AttestationData post-electra is committee-indifferent, and thus, the same attestation_data_root might be shared across different committees.

An aggregator is only interested in attestations in its corresponding committee. getAggregatedAttestationV1 might aggregate attestations from multiple committees since committee_index is not in its params.

This PR adds getAggregatedAttestationV2 to aggregate attestations given attestation_data_root + slot + committee_index.

Part of #445

[rkapka]

Adding to @nflaig 's comments, if we make committee index required then this endpoint will not be backwards compatible and validator clients will have to be aware of both.