Add support for querying Beams data inside the Service Worker

[jonathanlloyd]

There are times where it would be useful to be able to query Beams state inside the service worker. For example:

• You want to customize the notification based on the currently authenticated user
• Notification customizations must be done in the onNotificationReceived handler in your service worker
• Because you cannot currently access Beams state in the service worker, it is difficult to retrieve the currently authenticated user id

[daniel-shuy]

Implemented in #113