Motivation for this change

For some devices, power saving or MAC address randomization should be disabled to improve wifi stability.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

Relevant documentation for power saving.
It was not clear from the documentation whether power saving is enabled or disabled by default.
A nullable option that defaults to not writing the wifi.powersave property to NetworkManager.conf has been provided.