Improve offload.env defaults/comments regarding Spark settings

[nj1973]

We should do some testing with assorted Oracle and Spark/Dataproc versions and improve the comments/defaults for OFFLOAD_TRANSPORT_SPARK_PROPERTIES.

These pages will be useful:

• https://cloud.google.com/dataproc-serverless/docs/concepts/versions/spark-runtime-versions
• https://cloud.google.com/dataproc/docs/concepts/versioning/dataproc-release-2.2

For example

• We might want to figure out a matrix of ojdbc driver versions by database and Java version.
• The Avro package version 3.5.0 works on Dataproc Batches 1.1 and 1.2 and Dataproc 2.2. But not on earlier Dataprocs, perhaps our default should be 3.5.0 but with a comment regarding older Dataproc versions.
• Oracle wallet example/recommendations, perhaps always advise to use the same versions as for a version that created the wallet? Example package addresses: com.oracle.database.security:oraclepki:19.22.0.0,com.oracle.database.security:osdt_cert:19.22.0.0,com.oracle.database.security:osdt_core:19.22.0.0