Scoru: Store multiple cemented commitments in context

Context

In order to execute an outbox message against a cemented commitment that is not necessarily the last, we have to keep multiple cemented commitments for a rollup in context. This is a preparatory work for !5741 (merged). It closes #3464 (closed)

Manually testing the MR

dune exec src/proto_alpha/lib_protocol/test/unit/main.exe \
      -- test "^\[Unit\] Sc_rollup_storage.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