[WIP] CUDF Interchange Format

[mtjrider]

This PR adds initial support for data exchange between cudf and xgboost. It is matched by the XGB CUDF Integration PR.

• Add minimalist columnar data structure for numeric types, called gdf_interchange_column
• Add function to export a gdf_column type to gdf_interchange_column type
• Add C API for interchange
• Add C++ API for Google Tests for the above
• Add Python cuDF Dataframe.to_interchange(...) and Dataframe.from_interchange(...)
• Add pytests for the above

[RAMitchell]

This format could be used for interop with LightGBM/CatBoost so it should be carefully reviewed.

[harrism]

Obsolete, closing.