Explore widening support for xgboost models

[ascillitoe]

xgboost models under the sklearn interface are currently supportet by ClassifierDrift(..., backend='sklearn'). We should consider widening this support as @jklaise suggested in #642 (comment):

Very likely outside the scope of this PR, but is it true that we only support the sklearn interface for xgboost models where they can be used in the library (ClassifierDrift being the only place?) ?

Reason for asking is two-fold:

• should we support xgboost native format for detectors like ClassifierDrift?
• the native format seems to have good json-based serialization: >https://xgboost.readthedocs.io/en/stable/tutorials/saving_model.html

On a tangential note, we could also consider expanding support for sklearn/xgboost preprocessors.