Scoru: Extend Rollup Management Protocol with outbox type

Context

Addresses #2899 (closed).

Extends Sc_rollup_management_protocol with type outbox_message. This type represents the outbox of a SCORU and we provide a protocol level function outbox_message_of_bytes for parsing outbox messages into a type value.

Manually testing the MR

dune exec src/proto_alpha/lib_protocol/test/unit/main.exe -- test "^\[Unit\] sc rollup management protocol$"

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