Proto: add Operation.bls_mode_unsigned_encoding

What

This MR introduces Operation.bls_mode_unsigned_encoding, a dedicated operation encoding for signing and verifying attestation signatures with BLS (PoP) keys. In this encoding, the signed payload omits slots to enable BLS proof-of-possession aggregation.

This MR only adds the encoding and does not integrate it into any existing logic.

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, necessitating a specialized encoding alongside the standard Operation.unsigned_encoding.

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