Add Keylightd derivation and module, make available to Framework 13 and Framework 16

[twoolie]

Description of changes

Adds a simple wrapper to build keylightd and provide it as a systemd service. Has options to set the brightness and timeout, and control the power light.

This is a huge help when watching videos in dark rooms when the non-dimmable power light can be quite distracting.

Module is only imported for Framework 13 and 16 as the Framework 12 does not have a keyboard backlight.

Things done

• Tested the changes in your own NixOS Configuration
• Tested the changes end-to-end by using your fork of nixos-hardware and
  importing it via <nixos-hardware> or Flake input

[dantefromhell]

There's also https://gitlab.com/mamarley/keylightc which seems to be maintained more regularly, but it does not support changing the light status of the power button from what I can see.

[JohnAZoidberg]

This looks like it should go into nixpkgs not nixos-hardware. It's a package and a service that a user can enable/install if they like.

[twoolie]

I'm not against upstreaming the package, as framework-tool is already in nixpkgs.

I would contend that the module that exposes "hardware.framework.keylightd" should live here with the rest of the "hardware.framework.*" options, given that it only works with this particular framework hardware.