[8.x](backport #41289) Only watch metadata for ReplicaSets in metricbeat k8s module

[mergify]

Proposed commit message

Use metadata watchers for ReplicaSets in the metricbeats Kubernetes module. This is a similar change to #41100, with two major differences:

• State metrics for ReplicaSets may actually need all the metadata, so our transform function keeps it.
• Due to the way the update handler function switches over the resource type, our resource needs to have TypeMeta present. The informer doesn't set this, so we need to do it ourselves.

A decent chunk of the changes is just passing around a metadata-only k8s client.

For more details see elastic/elastic-agent#5623.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

You need a full K8s cluster, unfortunately. I've tested it using the default elastic-agent standalone manifest, by building a custom container image and loading it into a local kind cluster.

Related issues

• Relates Agent and beats store too much ReplicaSet data in K8s elastic-agent#5623

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This is an automatic backport of pull request #41289 done by [Mergify](https://mergify.com).

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @swiatekm? 🙏