[translation] consider poller design

[kristapratico]

For beta 1, we released a non-poller design that creates a translation job and only polls the service if the user calls the helper method wait_until_done with the job ID. For beta 2, we'll revisit this design and consider a few options:

1. Stick with current design
2. Replace design with non-live poller implementation (still probably keep wait_until_done)
3. Add a begin method that returns a poller (live or not live) in addition to having the create_job / wait_until_done methods
4. Stream documents from the list_all_document_statuses