make k8s ingestion core

[georgew5656]

Makes kubernetes-overlord-extensions a core extension.

Description

I think it makes sense to make this extension (k8s based ingestion) a core extension.

• It has significant unit test coverage and I am aware of at least a few users using it in production.
• It also has features like a migration path to/from mm ingestion.

Release note

Make k8s ingestion a core extension

Key changed/added classes in this PR

• move kubernetes-overlord-extensions from extensions-contrib -> extensions-core

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.

[vtlim]

For docs, can we add a redirect for other sources that link to the old location? For example:

druid/website/redirects.js

Lines 293 to 296 in 12e88b7

	{
	"from": "/docs/latest/development/extensions-contrib/google.html",
	"to": "/docs/latest/development/extensions-core/google"
	},