Dal/Rollup node: download block receipt and check slot availability

Context

The rollup node needs to track endorsed slots from the L1 tezos-node, and to recover the slot headers that will need to be downloaded from the Dal node. This MR depends on !6033 (merged) and is part of #3166 (closed). It is a pre-requisite for !6174 (merged)

Todo:

• tezt to check that the right list of endorsed slot headers is returned by the rollup node.

Closes: #3489 (closed)

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