Add v2 attestation APIs to support EIP-7549

[nflaig]

It is required to add v2 attestation APIs to support EIP-7549

From ethereum/beacon-APIs#447

• GET /eth/v2/validator/aggregate_attestation

From ethereum/beacon-APIs#448

• GET /eth/v2/beacon/blocks/{block_id}/attestations
• GET /eth/v2/beacon/pool/attestations
• POST /eth/v2/beacon/pool/attestations
• GET /eth/v2/beacon/pool/attester_slashings
• POST /eth/v2/beacon/pool/attester_slashings

From ethereum/beacon-APIs#456

• POST /eth/v2/validator/aggregate_and_proofs

From ethereum/beacon-APIs#459

• GET /eth/v2/events or #7026

[nflaig]

@nazarhussain in case you haven't started working on this, I think it makes sense to wait with this implementation until main spec PRs are merged as we don't need this for devnet-1. This allows to run against master branch of beacon-api spec to make sure the implementation is correct, this saves a lot of effort in reviewing the changes in detail manually.