[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 接口，并阅读现有的 getLimit() 和 getHeadRoom() API，了解它们的约定。跟踪它们的实现和测试，然后以一致的方式公开 allocator 的 init 预留，并验证相关的接口和实现测试。

由索引模型根据 Issue 内容生成。