Proto: add storage for recorded applied messages

Context

Address #2870 (closed). This MR adds the storage layer for keeping track of processed Layer 2 messages. Those are messages added to a SCORU's outbox.

An API in Alpha_context.Sc_rollup is provided for:

• Recording applied messages per rollup, level and message index.
• Check whether a message for a specific rollup, level and message index has been applied/recorded.

val record_applied_message :
  context -> t -> Raw_level.t -> message_index:int -> (Z.t * context) tzresult Lwt.t

Not that this API itself does not enforce garbage collection. Instead gc will be enforced by limiting the number of committed states and offer an end-point that removes all recorded messages for an expired level. This is mirroring the TORU design.

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