This integrates the Interaction feature transformer with SparkR R formula support (i.e. support `:`).

To generate reasonable ML attribute names for feature interactions, it was necessary to add the ability to read attribute the original attribute names back from `StructField`, and also to specify custom group prefixes in `VectorAssembler`. This also has the side-benefit of cleaning up the double-underscores in the attributes generated for non-interaction terms.

mengxr

Author: Eric Liang <ekl@databricks.com>

Closes #8830 from ericl/interaction-2.