[iOS] Add iCloud sync for session duration

Consider a CloudKit solution for backing up / syncing session-start timestamp and session duration.

Major concern: this work takes data that previously only existed on the user’s device, and puts it in the cloud.

  • Encrypt the data
  • Be very explicit about what’s synced to iCloud
  • Opt-in by default
  • Give user full access to view data that’s being synced
  • Give user full access to remove data from iCloud