DAL/Node: merge get_dal_content_of_attestations and get_attestation_operations

Follow up from @eugenz remark !16223 (comment 2298150055)

What

Merge two functions (get_dal_content_of_attestations and get_attestation_operations) in the DAL plugin registration.

Why

They were redundant.

How

By defining a new function get_attestations that return enough information to supply both functions.

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