[Java] Allow a user to provide an alternative "chunk" allocator

Right now, the Arrow Java libraries have two options:
- Have accounted memory using the Netty allocator.
- Have unaccounted memory using your own allocator.

I'd like to add a third option where you can use the existing accounting but decide where the chunks of memory come from.

**Reporter**: [Jacques Nadeau](https://issues.apache.org/jira/browse/ARROW-7198) / @jacques-n

_{**Note**: *This issue was originally created as [ARROW-7198](https://issues.apache.org/jira/browse/ARROW-7198). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

No files, tests, or entry points are named. Start by locating the existing Netty-backed and user-provided allocator paths in the Arrow Java libraries, then determine how a configurable chunk source can preserve memory accounting; done means the third allocation option works without losing accounting.

Written by the indexing model from the issue text.