[SPARK-16581][SPARKR] Make JVM backend calling functions public

## What changes were proposed in this pull request?

This change exposes a public API in SparkR to create objects, call methods on the Spark driver JVM

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

Unit tests, CRAN checks

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #14775 from shivaram/sparkr-java-api.