Draft: Dal/Scoru: Enable rollup node to call RPCs of Dal nodes

Context

For the rollup node to be available to download slots for the dal, it must connect to a Dal Node (possibly more than one in the future). This MR is part of #3166 (closed)

This MR enables a rollup node to specify a DAL node in its configuration. This MR is a pre-requisite for !6174 (merged)

Manually testing the MR

No functionality is introduced in the rollup node in the MR. The connection to the dal node (and the actual download of data) is tested in !6174 (merged)

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