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GH-37728: [Java] Add methods to get an Iterable for a ValueVector (#41895)

### Rationale for this change

Simplify validating the values in a `ValueVector` in unit tests.

### What changes are included in this PR?

Methods for creating an `Iterable` and `Iterator` for a `ValueVector`. Also updated some unit tests to use the new methods.

### Are these changes tested?

Some unit tests were updated.

### Are there any user-facing changes?

The new methods are publicly available in the `ValueVectorUtility` class.
* GitHub Issue: #37728

Authored-by: Norman Jordan <norman.jordan@improving.com>
Signed-off-by: David Li <li.davidm96@gmail.com>