[FSTORE-1461] Refactor Kafka out of python engine

[vatj]

This PR adds/fixes/changes...

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

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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)

[aversey]

engine/test_python.py and test_feature_group_writer.py should be fixed. They fail attempting to mock _init_kafka_resources and _init_kafka_consumer at the moment. Should be enough to change hsfs.engine.python.Engine._init_kafka_resources and hsfs.engine.python.Engine._init_kafka_consumer in mocks to hsfs.core.kafka_engine.init_kafka_resources and hsfs.core.kafka_engine.init_kafka_consumer.