[FEA] Example of RAPIDS UDF using cudf Java APIs

[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
A simple example Maven project can be added as a sub-project in the repo. If the example is very useful in practice, we could consider adding it to the sql-plugin project directly so it is available for use out-of-the-box with the existing plugin setup instructions rather than needing to build and use an additional jar.

Describe alternatives you've considered
If desired we could move the example into a completely separate repository. However the amount of code involved should be quite small and build quickly, and it would be easier to find and reference in the documentation if its part of the repository.