Proto,tx_rollup: Refactor the [state] type of [Tx_rollup_repr]

Context

This is a subpart of !4017 (merged), which has been extracted to reduce the size of the MR.

The changes in this MR should not be too controversial, except maybe the use of Not_found in the RPC (but I’ve added a comment to explain it).

The commit could have been split, if necessary I can do it at some point.

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR