Scoru/Rollup node: store slot headers

Context

This is required to enable the execution of messages coming from Dal slots in the rollup node PVM. See issue #3166 (closed) In this MR, support for storing slot headers for a blocks is added to the rollup node. The slot metadata <slot_metadata> for slot index i of block with hash block_hash is stored as a list of bindings (<i>, <slot_metadata>) at the Irmin path /dal/slot_headers/<block_hash>.

Closes issue: #3507 (closed)

Manually testing the MR

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