Add support for CatBoostRegressor

[krishanbhasin-gc]

Following #1403, it would be great to also support the CatBoostRegressor (and subsequently CatBoostRanker) model types.

From the linked PR:

Q:

Looking ahead to adding support for the Regressor and Ranker models, given catboost/catboost#2504 we can't use the same approach as XGBoost (using a try-catch).
It might be worth creating 3 separate MLModel child classes, one for each of Classifier, Regressor, and Ranker instead. Could this fit the design approach of the project?

A:

On what you mention around classifiers vs regressors, I think we should land this one as a first iteration and then open an issue to add support for extra types of CatBoost models. It's a shame the framework itself doesn't give us any way to differentiate between artefacts - in that case, we may need to add some extra config field to the model-settings.json to select between model types.