feat: Pass through individual channel configuration

[lpchaim]

While looking for a way to achieve per host nixpkgs configurations within snowfall a while ago (see #106), I found out it (very understandably) wasn't a supported use case. I've since familiarized myself some more with both nix and snowfall, so I figured I'd try and give solving that issue cleanly a shot. Hopefully the solution I came up with makes sense!

Essentially, channel-specific configuration is now passed through to flake-utils-plus.mkFlake, allowing channels to have distinct settings and making so individual hosts can have their channel changed. Additionally, the same overlays that were once applied only to the default nixpkgs channel are now also applied to any other user-specified channels automatically.

As a demonstration, the following is a minimal working snippet from my personal flake showcasing how I manage to enable cuda support for my one host with a NVIDIA GPU without affecting the other hosts.

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    nixpkgs-cuda.follows = "nixpkgs";
    snowfall-lib.url = "github:snowfallorg/lib";
  };

  outputs = inputs @ {self, ...}:
    snowfallLib.mkFlake {
      inherit inputs;
      src = ./.;

      channels-config = {
        allowUnfree = true;
      };
      channels.nixpkgs-cuda.config = {
        allowUnfree = true;
        cudaSupport = true;
      };

      systems.hosts.desktop.channelName = "nixpkgs-cuda";
    }
}