[jni] Add deep conversion utils

[imshubham22apr-gif]

Summary

This PR adds deep conversion utilities for package:jni, following the established pattern in package:objective_c. These utilities simplify the process of converting between Dart collections/primitive types and their JNI counterparts.

Changes

• Recursive Converters: Added toJObject and toDartObject which handle nested List, Set, and Map structures.
• Array Extensions: New extension methods on Dart List for direct conversion to JNI primitive arrays (e.g., toJIntArray, toJDoubleArray).
• Object Array Support: Added toJObjectArray for converting typed lists of JObject.
• Unit Tests: Comprehensive tests added in test/converter_test.dart to verify deep conversion of basic types, nested collections, and arrays.

Related Issues

Fixes #3143

PR Checklist

• Reviewed the contributor guide.
• Added new tests to check the changes.
• PR solves the mentioned issue.

[liamappelbe]

@imshubham22apr-gif Thanks for the contribution. Just FYI #3143 is a follow up TODO for #3093, which hasn't landed yet. So I won't be able to review this PR until that one lands. Hopefully that will be sometime next week.