Proto/validate: refactor block finalization and add checks in partial validation mode

Context

The MR does some refactoring in the block finalization of validate.ml, and adds checks related to preendorsements in Partial_validation mode.

Manually testing the MR

CI

Checklist

• [N/A] Document the interface of any function added or modified (see the coding guidelines)
• [N/A] 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