Better support for (push) notifications in E2EE rooms

In E2EE rooms, we want to decrypt the message content when receiving a push notification.
For this, we'll need to do a sync from the service worker and decrypt the message, apply push rules and see if it still notifies.

This will entail:
- make it possible to load the hydrogen SDK from a service worker, load the session (potentially only for a single room) and do catchup syncs until we can decrypt or we've drained all the to_device messages.
- implement push rules locally or the new notification actions. This will also allow us to show notifications when having the app open on platforms that don't support push notifications.

Start by tracing how the Hydrogen SDK is loaded, how a session performs catchup syncs, and how push notifications are handled in the service worker. Done means the service worker can load the session, decrypt relevant messages, apply push rules or notification actions, and avoid notifying when rules suppress the event.

Written by the indexing model from the issue text.