Benchmarks: new helper module

Context

This request continues ongoing efforts on simplifying how to write benchmarks.

It creates a helper module in lib_benchmarks_proto that will be populated as we simplify the benchmarks in this directory, and that, in the end, could be the only Snoop-related interface seen by the benchmarks.

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