Etherlink/Kernel: Store block headers in block::promote

Stacked on !17000 (merged).

What

When stage 2 successfully leads to the creation of a new L2 block, this MR makes the block::produce function store the header of this block outside of the world state.

Why

• This will be used to avoid potentially costly block reads in a few places from stage 1 and early stage 2. In particular, in !16835 (merged) this will be used to avoid reading the current block to start building the next one.

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