Add TreeSHAP dependencies to Alibi-Explain runtime image

[adriangonz]

The TreeSHAP explainer needs access to the underlying model instance. Therefore, to ensure TreeSHAP works out-of-the-box, we'll need to ensure the following libraries are present on the Alibi Explain runtime image:

• XGBoost
• LightGBM
• CatBoost
• SKLearn

[ascillitoe]

https://github.com/SeldonIO/MLServer/pull/1279 adds XGBoost, LightGBM and SKLearn to the runtime deps. CatBoost is still TODO, since for consistency we might want to wait for an mlserver_catboost runtime to be added first.