[Java] ComplexWriter should initialize nested writers when container vector is already populated

It's possible and sometimes useful to use reuse a nested vector that was populated in a previous ComplexWriter. The new ComplexWriter should be aware of the fields that are present in the vector.

As it is right now, if a particular column were determined to be a specific type (or a union type), but the new writer finds a new type, the original type may be thrown out. What should happen is that the type should be promoted to union (or have a new subtype added to the union field).

**Reporter**: [Steven Phillips](https://issues.apache.org/jira/browse/ARROW-326) / @StevenMPhillips

_{**Note**: *This issue was originally created as [ARROW-326](https://issues.apache.org/jira/browse/ARROW-326). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

まず、ComplexWriter と、既存のコンテナ・ベクターからネストされた writer を初期化するコードを見つけます。Issue で説明されている、値が格納されたベクターのケースを再現し、その後、既存の型が保持され、新しい型によってフィールドが union に昇格するかサブタイプが追加され、以前の型が失われないことを確認します。

索引モデルが issue の本文から書いたものです。