feat(awgen): add AmneziaWG configuration generator package

Package alex/awgen (offline, stateless Rust CLI for AmneziaWG configs) from
the upstream Gitea repository. Source is fetched with fetchFromGitea and
pinned to the v0.1 tag; the cargo dependency hash is pinned as well.

Also list the package in the README package table and project tree.

packages/awgen/default.nix

@@ -0,0 +1,5 @@
+{
+  pkgs,
+  ...
+}:
+pkgs.callPackage ./package.nix { }