Added Nix development flake

[t3rp]

• Adds flake.nix to provide reproducible dev shells/easier dependency management
• Installs Vagrant plugins and other dependencies from the GOAD documentation
• Exposes dev shells for local providers: libvirt, virtualbox, and vmware
• I did not add flake.nix to the PR, it is included below

Usage:

nix develop .#libvirt
nix develop .#virtualbox
nix develop .#vmware

Tested:

• Tested on x86_64-linux
• Libvirt/VMware/Virtualbox must be installed

Flake.lock:

{
  "nodes": {
    "nixpkgs": {
      "locked": {
        "lastModified": 1768395095,
        "narHash": "sha256-ZhuYJbwbZT32QA95tSkXd9zXHcdZj90EzHpEXBMabaw=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "13868c071cc73a5e9f610c47d7bb08e5da64fdd5",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "ref": "nixpkgs-unstable",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "nixpkgs": "nixpkgs"
      }
    }
  },
  "root": "root",
  "version": 7
}