Proto: change all_bakers_attest feature flag to an activation level

What

When changing behavior for attestations, there will be a period of time where both blocks with committee and blocks with every baker will coexist. Since they need to be handled differently, we need to know when the switch was turned on.

Why

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