[xgboost] Add xgboost transformation in caisar.

lib/xgboost/bin/parse_xgboost.ml

lib/xgboost/predict.mli

lib/xgboost/tree.ml

lib/xgboost/tree.mli

src/convert_xgboost.ml

src/convert_xgboost.mli

src/dataset.mli

src/dune

src/main.ml

src/verification.mli

tests/dune

tests/xgboost.t