[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.*}

首先定位 ComplexWriter 以及从现有容器向量初始化嵌套 writer 的代码。复现 issue 中描述的已填充向量场景，然后验证现有类型得到保留，并且新类型会将字段提升为 union 或添加 subtype，同时不会丢失之前的类型。

由索引模型根据 Issue 内容生成。