Truly unchecked getters

### Describe the enhancement requested

Right now the library integrating with arrow-java needs to resort to hacks of setting environment variable or system property at runtime if it knows its usages of arrow-java can omit null and bounds checking. However, that is not true for all of the usages in the given process leading to invalid data reads or segfaults.

To avoid this pitfall I suggest we add getUnchecked apis that NEVER do any null and bound checking but put the responsibility on the caller to guard against their possibility. For example in apache iceberg https://github.com/apache/iceberg/blob/23b5ce8eeca2d894c7973e19caa5c5ec02b4e4b8/spark/v4.1/spark/src/main/java/org/apache/iceberg/spark/data/vectorized/VectorizedSparkParquetReaders.java#L44-L51

These apis would make it easy to mix libraries that use arrow as an implementation detail along with user code that operates directly on arrow arrays

まず、既存の Arrow 配列 getter API と、リンク先の行にある Iceberg の VectorizedSparkParquetReaders.java でのチェックなしアクセスの使用状況を確認してください。Issue には arrow-java のファイルやテストが挙げられていないため、まず影響を受ける getter ファミリーとテスト箇所を特定してください。要求されたチェックなし API が一貫して利用でき、呼び出し側が責任を負う動作がテストでカバーされていれば完了です。

索引モデルが issue の本文から書いたものです。