Eliminate apimachinery dependency in the api module

[mozesl-nokia]

This PR removes the dependency on apimachinery by vendoring in the parts relevant for kpt. This is part of an effort to reduce the "dependency footprint" of kpt modules.

Needs a follow-up where this new version of the api module is adopted by the main kpt module (already ready from me).

Feel free to suggest additional parts of the GVK interface to include, I only added what kpt and the function SDK need.

Required by: kptdev/krm-functions-sdk#762

AI disclosure: Cursor's composer 2.5 was used to create the cut down files in the new schema package.

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[Copilot]

Pull request overview

This PR introduces a small, vendored-in schema package inside the api module to replace the k8s.io/apimachinery/pkg/runtime/schema dependency, reducing the api module’s dependency footprint while preserving the Group/Version/Kind types kpt needs.

Changes:

• Added api/schema/v1 with local GroupVersion, GroupKind, and GroupVersionKind types + helpers (ported from apimachinery) and accompanying tests.
• Updated api types to import the new local schema package instead of k8s.io/apimachinery/pkg/runtime/schema.
• Removed k8s.io/apimachinery (and related indirect deps) from api/go.mod / api/go.sum.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
api/schema/v1/group_version.go	Adds local GVK/GV/GK types and parsing helpers (ported from apimachinery).
api/schema/v1/group_version_test.go	Adds unit tests for the new schema helpers.
api/schema/doc.go	Documents the new schema package purpose (no apimachinery dependency).
api/resourcegroup/v1alpha1/types.go	Switches ResourceGroup GVK helper to local schema types.
api/kptfile/v1/types.go	Switches Kptfile GVK helper to local schema types.
api/fnresult/v1/types.go	Switches FunctionResultList GVK helper to local schema types.
api/go.mod	Removes direct apimachinery dependency; updates indirect deps.
api/go.sum	Drops apimachinery-related sums and other no-longer-needed entries.

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