[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.*}

Inizia individuando l’interfaccia Java BufferAllocator e leggi le API esistenti getLimit() e getHeadRoom() per comprenderne le convenzioni. Traccia le relative implementazioni e i test, poi esponi in modo coerente la prenotazione init dell’allocator e verifica i test pertinenti dell’interfaccia e dell’implementazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.