Update FedProx baseline

[jafermarq]

After the FedProx baseline was added in the SoR format, some formatting checks and tests were introduced. Those were never ran against FedProx. This PR ensures tests pass. To achieve this:

• The pyproject.toml has been updated with the new content (that already is part of that in the EXTENDED_README.md in the template baseline)
• Then, the formatting script and tests were run and pass (except one --> see comment below)
• Fixed a few typos

Once this is merged, progress can continue with #2210 and get it merged.

[jafermarq]

This line will trigger a mypy error since back in flwr=1.3 (the version pinned for SoR) start_simulation expected the client_fn callable to return a Client (not including NumpyClient). @danieljanes, what do you suggest doing here ? Should we bump flwr to 1.5 in pyproject.toml in a separate PR?

[jafermarq]

Will update once #2301 is merged

[jafermarq]

needs change from #2307

[jafermarq]

needs #2327

[charlesbvll]

LGTM!