Etherlink/Kernel/Stage 2: read block header instead of block

What

Stacked on !17122 (merged).

When constructing a block we need some information about the previous one. This MR switches the source of information to gather this information from the block itself to the block header.

Why

To avoid deserializing a potentially large block.

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