Split `nox` tools out to a new `frequenz-noxutil` repo

[llucax]

What's needed?

Move Nox sessions and helpers into frequenz-noxutil so downstream projects can import and reuse common nox session definitions and utility helpers.

Proposed solution

Files to move

• src/frequenz/repo/config/nox/* — utilities, common session definitions, and helpers used by the repository's noxfile.py.

Suggested new repository layout

• frequenz.noxutil (package)
  • __init__.py
  • sessions.py (or similar; exports helpers to build nox sessions)
  • small CLI or module-level helpers for session reuse