[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.*}

Commencez par localiser ComplexWriter et le code qui initialise des writers imbriqués à partir d’un vecteur de conteneur existant. Reproduisez le cas avec le vecteur rempli décrit dans l’issue, puis vérifiez que les types existants sont conservés et que les nouveaux types transforment le champ en une union ou ajoutent un sous-type sans perdre les types précédents.

Rédigé par le modèle d'indexation à partir du texte de l'issue.