Baker: update signature for BLS attestation with DAL content

This MR updates the signature for BLS attestations with DAL content as follows: signature_with_consensus_sk + (dal_content + 1) * signature_with_companion_sk.

This MR should be merged along with the protocol changes !17642 (merged).

Manually testing the MR

Tests are in a follow-up MR: !17843.

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