Draft: Fix endorsements of transition block

Context

The baker used to consider the migration block very specifically, in effect disabling consensus for this very block. In particular, this block was not endorsed.

While this was a good solution when transitioning from a consensus protocol (H) to another (I), it is not actually necessary when we stay within compatible consensus protocols, which is the case for the transition from protocol I to protocol J.

This fixes issue #2253 & concrete discussions from #2233 (closed)

Manually testing the MR

The newly added Tezt tests should pass but should fail without the patch.

dune exec tezt/tests/main.exe -- baker endorsing migration

The tests verify that all blocks, and most notably, the block after the migration block, include consensus operations.

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