Add tests of UDF examples from spark-rapids-examples to premerge tests

[jlowe]

The UDF examples jars have been moved to the spark-rapids-examples repository, but we should be testing against truly external UDFs in our premerge builds to verify we do not break this public API in a PR. The premerge build can install the plugin snapshot jar into the local m2 repository, pull the spark-rapids-examples repo, and build and run the UDF tests from that repo against that installed plugin snapshot. I don't think we need to build the full native builds, as that can be covered in nightly tests. The build and test of the Java-only UDF examples should be quick and have only a small impact on the premerge runtime.

[jlowe]

I think we can get sufficient, relatively timely coverage from the nightly UDF example tests, assuming they are setup against the latest plugin snapshot jar.

[res-life]

@NvTimLiu is checking the nightly pipeline which testing agaist the Snapshot dist jar. For per-merge pipeline, is it necessary?

[jlowe]

For per-merge pipeline, is it necessary?

I think we can go without for now, as long as the nightly UDF example tests are testing against the latest snapshot version of the plugin.