[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.

從 flight/flight-core/src/main/java/org/apache/arrow/flight/FlightClient.java 第 86 行附近開始，追蹤 FlightClient.Builder。確認預設情況下仍會建立子 allocator，而新的選項可以直接使用提供的 allocator。當相關的 Flight 用戶端測試涵蓋這兩種 allocator 模式時，即可完成。

由索引模型根據 Issue 內容生成。