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GH-37863: [Java] Add typed getters for StructVector (#37916)

### Rationale for this change
Add methods for getting child vectors as specific vector subtypes
for convenience.

### What changes are included in this PR?
Add child getters which let the caller specify the target vector type to StructVector.

### Are these changes tested?
Yes.

### Are there any user-facing changes?
No.
* Closes: #37863

Authored-by: James Duong <duong.james@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>