Etherlink/Kernel: Introduce block header

What

Stacked on !16996 (merged).

This MR introduces in the kernel a serializable structure called a "block header", which contains just enough information to build the successor of the block. For forward-compatibility with Tezos-compatibility and multichain, this structure is split into a generic part (block number and timestamp) and an EVM-specific part (block hash, receipts root hash, and transactions root hash).

Why

This structure will be used in the next MR (!16835 (merged)) to avoid reading the full block in stage 2.

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