[Java] Add getInitReservation() to BufferAllocator interface similar to getLimit(), getHeadRoom() APIs

For capturing additional information for debugging/profiling purposes, it will be useful to expose the init reservation for buffer allocator. 

I would encourage someone new to the community to do this.

**Reporter**: [Siddharth Teotia](https://issues.apache.org/jira/browse/ARROW-2001) / @siddharthteotia

_{**Note**: *This issue was originally created as [ARROW-2001](https://issues.apache.org/jira/browse/ARROW-2001). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

首先定位 Java BufferAllocator 介面，並閱讀現有的 getLimit() 與 getHeadRoom() API，了解它們的慣例。追蹤它們的實作與測試，接著一致地公開 allocator 的 init 預留，並驗證相關的介面與實作測試。

由索引模型根據 Issue 內容生成。