Feat: Intialize ScheduledThreadPoolExecutor with default worker threads

[uds5501]

Description

Updates workerThread calculation for ScheduledThreadPoolExecutor. It now follows the following logic:

• honor tuningConfig.workerThreads() if set.
• if auto scaler config / io config has max task counts specified, set worker threads to maxTaskCount/4.
• ensure at least 2 worker threads are spawned if the determined value (maxTaskCount/4) is too low.

Key changed/added classes in this PR

• SeekableStreamSupervisor

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• 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.

[kfaraz]

Changes look good.

Left a few style, javadoc suggestions.

+1 after CI passes

[kfaraz]

Left some final non-blocking comments.

Rest looks good 🚀

[uds5501]

Thanks for your review @kfaraz , I have addressed the final review comments as well.