nixos/hardware.fw-fanctrl: add package option and refactor using lib.attrsets.recursiveUpdate

Typically services have a `package` option, so it can be set externally
if users are running the stable version but want the package from
unstable, or devs want to test a package from their flake in production.
Really useful in many situations!

Also, the previous implementation was using `pkgs.runCommand` which is
discouraged due to
[IFD](https://nix.dev/manual/nix/2.26/language/import-from-derivation)
(import from derivation) leading to potential slowdowns during
evaluation. I opted for reading the json file and using
[lib.attrsets.recursiveUpdate](https://ryantm.github.io/nixpkgs/functions/library/attrsets/#function-library-lib.attrsets.recursiveUpdate)
to update the default values with the user provided ones.

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