DAL: Rollup node download endorsed slots from dal node

Context

This is the final MR required for implementing #3166 (closed). Endorsed slots are download and stored in the rollup node. Stored slots are optional values, where a value of None is used to represent a slot to which the rollup is subscribed to, but which has not been confirmed. This allows to distinguish between unsubscribed, unconfirmed and confirmed slots, which opens the way to implement any of the solutions proposed for how the PVM will interpret messages from the DAL.

This MR follows !6033 (merged), !6210 (merged), !6022 (merged) and !6272 (merged). It closes #3490 (closed) and #3515 (closed)

TODO:

• Tezt to check that slots are downloaded, and unconfirmed and confirmed slots are saved as such.

Manually testing the MR

make build-unreleased
dune exec tezt/tests/main.exe -- --file dal.ml

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