Bug 1641005 - Implement `mozISyncedExtensionStorageArea.fetchPendingSyncChanges()`. r=markh a=jcristau

This new method fetches pending synced changes from the extension
storage store, and passes them to `storage.onChanged` listeners.
This allows extensions that listen for these events to know when a
sync happened, which Kinto supported as well.

To guard against misuse, this method is implemented on a separate
`mozISyncedExtensionStorageArea` interface. To avoid multiple
inheritance (if `mozI{Synced, Configurable}ExtensionStorageArea` both
inherited from `mozIExtensionStorageArea`, which base method is called
when?), both of these internal-ish interfaces now inherit from
`nsISupports` instead.

Finally, because `fetchPendingSyncChanges` can return changes for
multiple extensions, `mozIExtensionStorageListener.onChanged` now takes
the affected extension ID as its first argument.

Differential Revision: https://phabricator.services.mozilla.com/D76976