Decouple Pusher ext from Pusher implementation

From https://discuss.flarum.org/d/27268-please-help-regarding-realtime-updates-of-posts/28

Several alternative drivers have emerged for realtime updates, including:

- blomstra/realtime
- kyrne/websocket

Each of these needs to completely re-implement the user-facing parts: updating discussions, posts, etc. If we refactored pusher into a "realtime" ext and had a well-documented interface, we could make Pusher a driver, and allow the substitution of custom drivers.

Start with the linked discussion and inspect how the existing Pusher extension is structured. The issue names no files, tests, or entry points, so first map the user-facing realtime behavior and the Pusher-specific implementation. Done would mean a documented realtime interface, Pusher as one driver, and support for substitutable custom drivers.

Written by the indexing model from the issue text.