[Flight client] Allow disabling the creation of a child allocator

### Describe the enhancement requested

`FlightClient` [always creates a child allocator](https://github.com/apache/arrow-java/blob/7cbf15994ae7c82fbcb178dded6b3f128219d9ad/flight/flight-core/src/main/java/org/apache/arrow/flight/FlightClient.java#L86). While this is good in the general case to track memory leaks, this prevents usage scenarios where vectors live longer than the Flight client.

What about adding a `FlightClient.Builder#createChildAllocator(boolean)` that would allow users to choose whether they want a child allocator (current behavior which will be the default) or directly use the provided one?

I can create a PR for this.

从 flight/flight-core/src/main/java/org/apache/arrow/flight/FlightClient.java 第 86 行附近开始，跟踪 FlightClient.Builder。验证默认情况下仍会创建子 allocator，同时新选项可以直接使用提供的 allocator。当相关的 Flight 客户端测试覆盖这两种 allocator 模式时，即可完成。

由索引模型根据 Issue 内容生成。