Refactor WindowOperatorQueryKit to use WindowStage class for representing different window stages

[Akshat-Jain]

Description

This PR refactors WindowOperatorQueryKit to use a new class WindowStage to represent the different window stages. Previously we were dealing with List<OperatorFactory> or List<List<OperatorFactory>>, and doing a bunch of instanceof checks, so this refactor makes it much more readable and cleaner.

This PR does not have any functional change.

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This PR has:

• [x] been self-reviewed.
• [ ] added documentation for new or modified features or behaviors.
• [ ] a release note entry in the PR description.
• [ ] added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• [ ] added or updated version, license, or notice information in licenses.yaml
• [ ] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• [ ] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• [ ] added integration tests.
• [ ] been tested in a test Druid cluster.