[formrecognizer] Update async methods to return AsyncLROPoller and support continuation token

[kristapratico]

Currently waiting on:

1. PR for azure-core: LRO continuation_token #10801
2. Work from autorest to support this, which is pending.

This will allow us to return AsyncLROPoller for our async methods and support continuation tokens. Because of the change in return type we will do a re-naming to align with other languages (feel free to spin-off on a different issue):

• sync method begin_train_model -> begin_training
• async method train_model -> begin_training
• all async methods will now have begin prefix added to them