Proto/operation_repr: compare aggregated operations by committee size

What

Preattestations_aggregate and Attestations_aggregate operations are now compared by committee size instead of committee lexicographical order.

Why

• When deciding which operation to keep, the size of the committee makes is more relevant than the first elements of the committee.
• This makes the weight infos independant of the type of the committee elements, which will make it easier to change the Attestations_aggregate committee to (slot * dal_slots option) list.

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