Proto: minor refactor for conflicts handling in double-attesting validation

What

Minor refactoring in validation.ml

Why

This change was originally part of !15244 (merged) as the addition of a new consensus operation (Attestations_aggregate) added nondesirable case in some pattern matchings. To facilitate the review process, this part has been split into this separate MR.

In the previous implementation, some functions took a 'kind Kind.consensus operation as a parameter, which became problematic when introducing a new consensus operation (Attestations_aggregate), as it is not possible to produce a conflict key for this operation.

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