XGBoost evaluator

[theadactyl]

While TFX can train any model (as long as you can feed it the data), evaluator needs a custom extractor to run Evaluation jobs. XGBoost evaluator would basically reuse the Evaluator executor but instead provide with the extractor for XGBoost internally.

[theadactyl]

cc @casassg

[rcrowe-google]

Could be an example or a library, to make working with XGBoost easier.

[rcrowe-google]

Proposal being reviewed by Google team