Create a "Notification service" specific client which supports only a subset of the whole sync logic

[poljar]

As described in #1928, supporting notifications in encrypted rooms on iOS would require us to do a /sync in the notification process.

The first step towards this would be to have a specialized client object which does the correct and minimal /sync setup for the service to render a specific push notification.

Additionally, this specialized client object would need to agree with the client running in the main process on who is responsible for the sync of the to-device events.