[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.*}

Beginne damit, ComplexWriter und den Code zu finden, der verschachtelte Writer aus einem vorhandenen Container-Vektor initialisiert. Stelle den im Issue beschriebenen Fall mit einem befüllten Vektor nach und überprüfe anschließend, dass vorhandene Typen erhalten bleiben und neue Typen das Feld zu einer Union hochstufen oder einen Subtyp hinzufügen, ohne zuvor vorhandene Typen zu verlieren.

Vom Indexierungsmodell aus dem Issue-Text verfasst.