Kubernetes executionconfig API should return effective config

[GWphua]

Description

#18591 introduced the capability to change the max number of K8s tasks created by Overlord, and we can track the values using /druid/indexer/v1/k8s/taskrunner/executionconfig` API.

Execution Config Returns Static Config by Default

Currently, calling GET on the /druid/indexer/v1/k8s/taskrunner/executionconfig API will return an empty payload if we did not submit any dynamic configs via POST on the same API. Users who depend on the API to retrieve the currently effective K8s task runner information will not see the information before the first dynamic config change is submitted.

Changed the API to return the effective config, so the users will always see what is currently being used.

Release note

Users now can retrieve the Kubernetes default configuration via GET on /druid/indexer/v1/k8s/taskrunner/executionconfig when there's no dynamic configs submitted.

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This PR has:

• been self-reviewed.
• a release note entry in the PR description.
• 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.
• been tested in a test Druid cluster.