Support unread thread notifications in SSS mode

[dbkr]

This adds support for reading unread_thread_notifications in SSS mode as we do in regular sync mode.

I've proposed adding this to the SSS MSC and I've implemented it in Synapse.

Also factors out the block of code that does it as it's common to sync v2 and SSS (but too sync specific to put in room, plus room.ts is already huge).

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).