Implement Union validity bitmap changes from ARROW-9222

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-9324

The implementation of Union in arrow has evolved from the current arrow-rs implementation. We need to update arrow-rs to follow the updated Arrow spec, including the updated validity bitmap changes from https://issues.apache.org/jira/browse/ARROW-9222

Related: Null/Sparse Union Array support: #814

As part of implementing the new Union handling, we should also update the checks in ArrayData::validate added in #810

[paddyhoran]

Context: https://lists.apache.org/thread.html/r5664e13d0a28ef36f6244864b3b96217d082bdfafb29e866cbefbfa4%40%3Cdev.arrow.apache.org%3E

[alamb]

FYI @paddyhoran I have a proposed implementation in #885 (look at the last commit)

[paddyhoran]

Excellent, I’ll find some time to take a look. Thanks @alamb