Rewrite webextension shim in Rust

The web extension shim translates between the native messaging protocol and the D-Bus API for credentialsd. It's currently written in Python using the dbus-next Python package.

We target Ubuntu 22.04 as our oldest supported OS, but the dbus-next package isn't packaged until 25.04. To make it easier for people to test using the extension, it may be worth rewriting this in Rust.

However, part of the purpose of using a different language, namely Python, was to catch discrepancies between docs and the implementation. If we do rewrite it, it might be good to use a different library than zbus (like dbus-rs) or not to use the proxy macros from zbus and craft messages manually. This might be overkill, and maybe we just need to write better integration tests.

webext/app から始め、native messaging と D-Bus の動作を文書化されたインターフェースと比較します。Rust での書き直し、またはより優れた統合テストによって Ubuntu 22.04 のパッケージング制約に対処できるかを評価し、利用できない dbus-next パッケージに依存せずに shim の動作を維持することを完了条件とします。