Save model in `XGBoost`

[daavoo]

Extend the existing XGBoost integration (dvc.xgb) to include model saving functionality. Extend test_xgboost.py to cover this feature.

Ref:

• Official XGBoost docs on saving models: https://xgboost.readthedocs.io/en/stable/tutorials/saving_model.html & https://mljar.com/blog/xgboost-save-load-python/
• Similar extension added to keras integration: Save checkpoint for Keras #69