Add support for fitting estimators on dask bags

The estimators that can handle dask bags should set the bob_fit_supports_dask_bag as True. This commit also includes

• Adds a new tag: bob_fit_supports_dask_bag
• Adds a new tag: bob_checkpoint_features for when you want to always avoid checkpointing features for a specific estimator.
• Expose dask_tags, get_bob_tags in the main API
• The SampleWrapper was modified to support bob_fit_supports_dask_bag
• The CheckpointWrapper now loads estimators without losing references correctly.