Proto: SCORU: extend output proofs with oubox-level

Context

Addresses missing part #2927 (closed). Follow up from !5478 (merged):

• Add ouput_proof_encoding as we need to decode output proofs (1b180bb5)
• Also introduce outbox_level field (70989e03)
• Minor renaming/refactoring

Manually testing the MR

CI, including:

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

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