[Dal] Dal node: Endpoint for returning slot contents as a list of segments

Context

The rollup node need to download the slots from the rollup node as a list of pages of a predetermined size. This MR defines an RPC endpoint in the Dal node to return the slots in this format. It isolates some commits previously belonging to !6174 (merged).

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