[Java] Investigate adding a getUnsafe method to vectors

As per: https://github.com/apache/arrow/pull/7095#issuecomment-625579459

**Reporter**: [Ryan Murray](https://issues.apache.org/jira/browse/ARROW-8738) / @rymurr

<sub>**Note**: *This issue was originally created as [ARROW-8738](https://issues.apache.org/jira/browse/ARROW-8738). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*</sub>