[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.*}

Comece localizando a interface Java BufferAllocator e leia as APIs existentes getLimit() e getHeadRoom() para entender suas convenções. Rastreie suas implementações e testes, depois exponha de forma consistente a reserva init do allocator e verifique os testes relevantes da interface e da implementação.

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