Configuration hot-reloading

[aumetra]

Is your feature request related to a problem? Please describe.

Right now admins need to fully restart Kitsune to roll out simple configuration changes. This can result in downtime and maybe even lost posts from a user-side (depending on how reliable the clients are).

Describe the solution you'd like

notify-based reloading of the configuration file. The configuration file will then be replaced with the new one. For all state-relevant replacement things, we could trigger a dynamic rebuilding of the axum state, and replace it via ArcSwap.

Describe alternatives you've considered

N/A