[xgboost] Add xgboost reading and predicting library.

lib/xgboost/bin/dune

lib/xgboost/bin/parse_xgboost.ml

lib/xgboost/dune

lib/xgboost/example/agaricus.test.svm

lib/xgboost/example/agaricus.txt.train

lib/xgboost/example/basic_walkthrough.ipynb

lib/xgboost/example/dune

lib/xgboost/example/logistic.t

lib/xgboost/example/model-0.json

lib/xgboost/input.ml

lib/xgboost/input.mli

lib/xgboost/model.schema

lib/xgboost/parser.ml

lib/xgboost/predict.ml

lib/xgboost/predict.mli