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GH-40930: [Java] Implement a function to retrieve reference buffers in StringView (#41796)

### Rationale for this change

This PR includes a minor changes to the `getBuffers` method in `BaseVariableWidthViewVector`. 

### What changes are included in this PR?

Previously the fixed buffers were the only ones returned from this method because of lack of clarity in the initial implementation stage. In this PR, it includes the variadic buffers to the result. A test case has also being added. 

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* GitHub Issue: #40930

Authored-by: Vibhatha Lakmal Abeykoon <vibhatha@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>