Return id(s) when calling Index::addItem and Index::addItems in Java bindings

[stephen29xie]

Description

In C++ and Python, the id of the item(s) are returned when they are added to the index. In Java, these methods return void. This PR brings these to parity.

Related Issues

#84

Changes Made

Java

• Return id(s) when calling Index::addItem and Index::addItems in Java bindings.
• Update docs
• Update tests

Testing

Checklist

• My code follows the code style of this project.
• I have added and/or updated appropriate documentation (if applicable).
• All new and existing tests pass locally with these changes.
• I have run static code analysis (if available) and resolved any issues.
• I have considered backward compatibility (if applicable).
• I have confirmed that this PR does not introduce any security vulnerabilities.

Additional Comments

[psobot]

LGTM with two suggestions. Thanks!