Add Linode host configuration and WireGuard setup

- Introduced new configuration files for Linode host, including hardware configuration, toggles, and WireGuard settings.
- Updated flake.nix to include the new images.nix file for Linode image generation.
- Adjusted SSH key paths and secrets management for WireGuard to ensure proper integration with the new host setup.
- Enhanced firewall rules and NAT configuration for WireGuard to improve security and connectivity.

parts/images.nix

@@ -0,0 +1,8 @@
+{ inputs, ... }:
+{
+  flake.packages.x86_64-linux.linode-image = inputs.nixos-generators.nixosGenerate {
+    system = "x86_64-linux";
+    modules = [ inputs.self.nixosConfigurations.linode.config.system.build.toplevel.outPath ];
+    format = "linode";
+  };
+}