Adds aggregation/attestations package

[farazdagi]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

• All code implementing the new aggregation attestation has been accumulated under aggregation and aggregation/attestation packages. Previous implementation aka naive aggregation code has also been moved here (w/o deleting it in original helpers/ directory).
• The next PR (Enables new aggregation implementation #6344), will be solely to replace all the references to the previous implementation with the current one.
• So, this PR is done to limit scope of changes:
  • It is safer to merge, as all changes are within aggregation/attestations and are not yet enabled
  • It is easier to review, as change is self-contained

Which issues(s) does this PR fix?

• Part of Optimize proposer attestation inclusion #4507

Other notes for review

• Introduces new flag (part of Feature Flag Tracking: attestationAggregationStrategy #6236)

[prestonvanloon]

Looks good!