add basic system tests for OpenLineage

[mobuchowski]

Added system tests to make sure our OpenLineage integration actually works as expected. Here's what's new:

• Built a VariableTransport that dumps OpenLineage events into Airflow Variables so we can check them later
• Created a test operator (OpenLineageTestOperator) that compares the events we got against what we expected
• Added a simple test DAG to show how it all works together
• Threw in some helper functions to make event validation less painful (checking UUIDs, dates, etc.)

The main idea is to have a reliable way to verify that we're sending the right OpenLineage events with the right format.
The test setup is pretty flexible - you can either define your expected events inline or keep them in JSON files, whatever works better for your use case.

[kacpermuda]

Looks good, leaving some nits - mostly naming or something similar.

[mobuchowski]

@Lee-W fixed all the typing/style comments.

[dstandish]

thanks @mobuchowski