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GH-38254: [Java] Add reusable buffer getters to char/binary vectors (#38266)

### Rationale for this change
Provide a way for a user to reuse a buffer when iterating over byte-array-based ValueVectors to avoid excessive
reallocations.

### What changes are included in this PR?
Add a reusable buffer interface that can be populated by character and binary vectors to avoid allocations when consuming vector content.

Optimize getObject() on VarCharVector/LargeVarCharVector to avoid an extra allocation of a byte array (copy from ArrowBuf directly to the resulting Text).

### Are these changes tested?

### Are there any user-facing changes?

Yes.

**This PR includes breaking changes to public APIs.**

* Closes: #38254

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