[Java] Support iterating a vector by ArrowBufPointer

[asfimport]

Provide the functionality to traverse a vector (fixed-width vector & variable-width vector) by an iterator. This is convenient for scenarios when accessing vector elements in sequence.

Reporter: Liya Fan / @liyafan82
Assignee: Liya Fan / @liyafan82

PRs and other links:

• GitHub Pull Request #4950

_{Note: This issue was originally created as ARROW-6031. Please see the migration documentation for further details.}

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 4950
#4950