Proto: change Int_percentage to hide its type

Depends on !11684 (merged)

What

This MR changes the name of the Percentage module in the protocol, while abstracting its type

Why

This is a refactoring MR that prepares for upcoming MRs for %Adaptive Issuance - P

How

The renaming is straightforward. The hiding type requires to expose a new function to_q, as well as comparison functions (only one is used at the moment). More constant percentages have been added for convenience.

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