DAL+Etherlink: configure the DAL slot index to use

Stacked on !13634 (merged).

What

Add a field in the Etherlink kernel to configure the DAL slot index to use. This field is currently not used.

Why

This is required to make the kernel reveal DAL pages from the configured slot index.

How

We set the DAL slot as a u8 in path /evm/dal_slot.

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