Etherlink/Kernel/Stage 2: refactoring block::produce

What

This is a pure refactoring MR enabled by !16274 (merged). It simplifies the block::produce function by factorizing the cases where the block in progress comes from the storage or from a blueprint.

Why

To avoid code duplication and ease maintenance.

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