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.