Etherlink/Kernel/Migration: initialize block header

Stacked on !17119 (merged).

What

This MR is a storage migration. It introduces storage version V27 starting from which the field /evm/block_header can be assumed to contain the BlockHeader of the current block.

Why

This lets the kernel access information about the current block without fetching the totality of the block from storage and without accessing the world state.

How

The field is initialized from the current block.

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