Implement openstf interface

The generic regressor interface can be found here: https://github.com/alliander-opensource/openstf/blob/main/openstf/model/regressors/regressor_interface.py

An example for the sklearn-compatible XGBoost regressor: https://github.com/alliander-opensource/openstf/blob/main/openstf/model/regressors/xgb.py

An example for our custom implemented Quantile-XGBoost regressor: https://github.com/alliander-opensource/openstf/blob/main/openstf/model/regressors/xgb_quantile.py