feat(nix): implement overlays

* setup repo structure
* configure overlay for 'kluctl' package first; once this works, MVP
  delivered
* since this is the initial commit on this feature, it's obviously
  broken and throwing the following error

```sh
error:
       … while evaluating a branch condition

         at /nix/store/7gmkl9bfx98djaamhym69wkj4r3sh1q6-source/lib/lists.nix:125:9:

          124|       fold' = n:
          125|         if n == len
             |         ^
          126|         then nul

       … while calling the 'length' builtin

         at /nix/store/7gmkl9bfx98djaamhym69wkj4r3sh1q6-source/lib/lists.nix:123:13:

          122|     let
          123|       len = length list;
             |             ^
          124|       fold' = n:

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: function 'anonymous lambda' called without required argument 'pkgs'

       at /nix/store/2cv47br877dpnc9p1cv3x95a27nsidb0-source/overlays/kluctl/default.nix:1:1:

            1| { pkgs, config, lib, ... }:
             | ^
            2|
```

Resolves: #9
Related:
Signed-off-by: Daniel-Andrei Minca <zealous_sterner.0e@icloud.com>

flake.nix

@@ -58,6 +58,7 @@
           sops-nix.homeManagerModules.sops
           ./hosts/common
           ./hosts/M-C02FX3JUML85
+          (args: { nixpkgs.overlays = import ./overlays args; } )
         ];
       };
     };

overlays/default.nix

@@ -0,0 +1,10 @@
+# import all nix files in the current folder, and execute them with args as parameters
+# The return value is a list of all execution results, which is the list of overlays
+
+args:
+# execute and import all overlay files in the current directory with the given args
+builtins.map
+  (f: (import (./. + "/${f}") args))  # execute and import the overlay file
+  (builtins.filter          # find all overlay files in the current directory
+    (f: f != "default.nix")
+    (builtins.attrNames (builtins.readDir ./.)))

overlays/kluctl/default.nix

@@ -0,0 +1,7 @@
+{ pkgs, config, lib, ... }:
+
+(self: super: {
+  kluctl = super.kluctl.override {
+    version = "2.25.0";
+  };
+})