feat: allow loading configurations from parent directories

[yaswanthkumar1995]

Fixes #6124

What this PR does

Allows the configurations field in kustomization.yaml to reference files in parent directories (e.g., configurations: [../kustomizeconfig.yaml]).

Previously, this was blocked by the RestrictionRootOnly load restrictor, forcing users to duplicate shared configuration files across every overlay.

Changes

• Add RestrictionRootAndAncestors load restrictor that permits files in root and ancestor directories, while still blocking sibling/unrelated paths
• Add WithLoadRestrictor method to FileLoader for per-field restriction overrides
• Apply relaxed restriction specifically to the configurations field in kusttarget.go
• Add unit tests for the new restrictor and integration test for parent-dir configurations

Why

Configuration files (nameReference, varReference, etc.) are metadata commonly shared across multiple kustomizations. Unlike resources or patches, they don't contain actual Kubernetes objects — they only define how kustomize should process fields. Requiring them to live under each overlay leads to duplication and drift.

How to test

# repo-root/kustomizeconfig.yaml  (shared config)
nameReference:
- kind: ConfigMap
  fieldSpecs:
  - kind: Deployment
    path: spec/template/spec/containers/env/valueFrom/configMapKeyRef/name

# repo-root/overlay/kustomization.yaml
configurations:
- ../kustomizeconfig.yaml
resources:
- deployment.yaml

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• ✅ login: yaswanthkumar1995 / name: Yaswanth Kumar (7f489e2)

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