[FSTORE-1800] Add support for pojo objects in flink client

[SirOibaf]

This PR adds/fixes/changes...

• please summarize your changes to the code
• and make sure to include all changes to user-facing APIs

JIRA Issue: -

Priority for Review: -

Related PRs: -

How Has This Been Tested?

• Unit Tests
• Integration Tests
• Manual Tests on VM

Checklist For The Assigned Reviewer:

- [ ] Checked if merge conflicts with master exist
- [ ] Checked if stylechecks for Java and Python pass
- [ ] Checked if all docstrings were added and/or updated appropriately
- [ ] Ran spellcheck on docstring
- [ ] Checked if guides & concepts need to be updated
- [ ] Checked if naming conventions for parameters and variables were followed
- [ ] Checked if private methods are properly declared and used
- [ ] Checked if hard-to-understand areas of code are commented
- [ ] Checked if tests are effective
- [ ] Built and deployed changes on dev VM and tested manually
- [x] (Checked if all type annotations were added and/or updated appropriately)

[Copilot]

Pull Request Overview

This PR adds support for POJO objects in the Flink client by refactoring POJO-to-Avro conversion logic. The changes centralize the conversion logic into a shared utility class and update the Flink engine to use this common implementation.

Key changes:

• Extract POJO-to-Avro conversion logic from Engine.java into a shared utility class
• Refactor Flink engine to use the centralized conversion logic
• Update version numbers across all modules to 3.9.0-RC22

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
java/hsfs/src/main/java/com/logicalclocks/hsfs/engine/PojoToAvroUtils.java	New utility class containing extracted POJO-to-Avro conversion logic
java/hsfs/src/main/java/com/logicalclocks/hsfs/engine/Engine.java	Refactored to use PojoToAvroUtils instead of inline conversion methods
java/flink/src/main/java/com/logicalclocks/hsfs/flink/engine/PojoToAvroRecord.java	Simplified to use PojoToAvroUtils for conversion
java/flink/src/main/java/com/logicalclocks/hsfs/flink/engine/FlinkEngine.java	Updated to pass schema strings instead of deserialized schemas
Multiple pom.xml files	Version bumps from 3.9.0-RC21 to 3.9.0-RC22
python/hsfs/version.py	Python version bump to match Java version