Add the ability to show as “busy” with apple’s new Focus API

https://developer.apple.com/forums/thread/682143

Focus modes can enable its "share focus status", or, very crudely, allow apps to read and get notified when focus mode is enabled, and the person may not get messages.

The above api allows for both reading upon authorisation, and getting an intent fired whenever focus status changes.

This can be ideal for sharing DnD status over matrix, via presence, with [MSC3026](https://github.com/matrix-org/matrix-doc/pull/3026) (or likewise), to announce such a presence automatically. A setting to automatically pick up on it via system settings and submit that status to a matrix server.