Proto/validate: update sig check for attestations_aggregate using DAL and companion keys

What

Updates the aggregation of public keys for the signature check of attestations_aggregate, to include companion keys weighted by the DAL content of each committee member (with weight zero when DAL is None).

This MR is on top of both !17642 (merged) and !17835 (merged). Only the last two commits belong to this 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