Proto: add aggregate_attestation feature flag

What

Add aggregate_attestation feature flag as a new protocol constant.

Why

In an effort to compress blocks footprint on disks we would like to compress the list of attestations in a block into a single aggregate one. This MR adds a feature flag that will allow us to keep the feature disabled until fully developped.

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR