Update KAL to support new guidance on required and optional fields

[JoelSpeed]

This enforces that all API fields are omitted when unset, and serialized correctly based on whether or not their zero value is a valid user choice.

This update aligns with updated guidance in the upstream API conventions, and downstream (see openshift/enhancements#1825)

This will remove the ability for APIs to be discoverable, by dropping omitempty, the motivation for this change is explained in the linked conventions update above.

[openshift-ci]

Hello @JoelSpeed! Some important instructions when contributing to openshift/api:
API design plays an important part in the user experience of OpenShift and as such API PRs are subject to a high level of scrutiny to ensure they follow our best practices. If you haven't already done so, please review the OpenShift API Conventions and ensure that your proposed changes are compliant. Following these conventions will help expedite the api review process for your PR.

[everettraven]

/lgtm

[openshift-ci]

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[openshift-ci]

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[ART PR BUILD NOTIFIER]

Distgit: ose-cluster-config-api
This PR has been included in build ose-cluster-config-api-container-v4.20.0-202508081715.p0.gc974eea.assembly.stream.el9.
All builds following this will include this PR.