[Java] Separate Netty Memory to its own module

[asfimport]

Finish the work started in ARROW-8230

Reporter: Ryan Murray / @rymurr
Assignee: Ryan Murray / @rymurr

Related issues:

• [Java] Remove Netty references from ArrowBuf and move Allocator out of vector package (is a child of)
• [Java] Document how to set netty/unsafe allocators (relates to)
• [Java] Remove Netty dependency from arrow-vector (is depended upon by)

PRs and other links:

• GitHub Pull Request apache/arrow#7619

_{Note: This issue was originally created as ARROW-9300. Please see the migration documentation for further details.}

[asfimport]

Bryan Cutler / @BryanCutler:
Issue resolved by pull request 7619
#7619