DAL+Etherlink: feature flag

Stacked on !13762 (merged).

What

This MR adds a feature-flag mechanism for the DAL intergration to the Etherlink kernel.

Why

To decorrelate the development of the integration of DAL in Etherlink from the activation of the DAL feature.

How

A field is added to the EVM kernel at the /evm/feature_flags/enable_dal path. When some value is set, the kernel is allowed to import data from the DAL. In this MR, the kernel logic is exactly the same when the feature flag is set.

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