Draft: feat: plugin integration [#1518]

This MR contains a prototype showcasing integration of a third-party plugin.

The pilot plugin used for this prototype is @eyo/mlaf. The changes necessary to make the plugin compatible with the proposed architecture can be found in this branch: https://gitlab.com/jensarps.adblock/mlaf/-/tree/feat/plugin-integration?ref_type=heads

The prototype uses the lifecycle hook architecture approach as outlined in this document. The implementation is extremely minimal, on purpose. It only covers the background context, it's not possible to register code to be included during build-time, and the only available lifecycle sates to hook into are during the extension startup.