DAL/RPC: update GET /commitments/<commitment>/headers

What

Updates the GET /commitments/<commitment>/headers RPC of the DAL node to

• Use slot_id instead of commitment,
• Return a single value (instead of a list), and
• Only return the status (instead of the complete slot header).

Why

This is a step toward indexing all RPCs with slot ids instead of commitments.

How

Most of the work of this MR consists in simplifying and updating the test using the RPC.

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