[FEA] Example of RAPIDS UDF using custom GPU code

[jlowe]

Is your feature request related to a problem? Please describe.
#1351 tracks implementing support for UDFs that provide a RAPIDS execution path. With that implementation should be working examples that can be used as a reference by users when implementing their own RAPIDS-accelerated UDFs.

Describe the solution you'd like
We will need to discuss whether this is more appropriate to be a separate repository or part of the current repository. Since custom GPU code needs to be built as part of the working example, there's a lot more involved in setting up the native build environment that is not needed for building any other code in this repository. Therefore it may be more appropriate to implement it in a separate repository. If it is implemented in the current repository there needs to be a Maven profile to control whether it is built.