Proto: attestation slots are no longer signed (under feature flag)

What

Under attestation_aggregate feature flag, use Operation.bls_mode_unsigned_encoding to check consensus operations signatures when providing a BLS signature.

Tezt and alpha baker are updated accordingly.

Why

To enable BLS proof-of-possession aggregation for consensus operation signatures, all signatures must be generated over the same message. This requirement applies only to consensus operations signed with BLS keys.

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