Proto: SCORU: add `level` to refutation game type

Context

A small MR that adds level into the refutation game type. This is the inbox_level of the commitment the game is refuting---it represents the furthest point in the inbox a tick in that refutation game can read.

This will be used by the refutation game proof verifier. It is being included in a separate MR here to help with implementation of the roll-up node's game-playing logic.

Manually testing the MR

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