feat(charts): add kube-gatekeeper Helm chart with initial configuration and documentation

[apetrovYa]

Description

Adds a new wrapper Helm chart kube-gatekeeper that wraps the official Gatekeeper chart from the Open Policy Agent project. Gatekeeper is a policy controller for Kubernetes that enforces custom resource-based policies (Constraint Templates and Constraints) and validates admission requests.

Changes:

• New chart charts/kube-gatekeeper with dependency on upstream gatekeeper (open-policy-agent/gatekeeper) v3.22.0-beta.0
• Upstream values exposed under spec following the kube-* wrapper pattern
• Chart.yaml, values.yaml, values.schema.json, README.md, .helmignore, Makefile, and release.json

No additional dependencies required; the chart pulls the upstream dependency from https://open-policy-agent.github.io/gatekeeper/charts.

Type of change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Chart Version Update

How Has This Been Tested?

• Chart structure and helm dependency update / helm template validated locally
• README and values follow existing wrapper chart patterns (e.g. kube-oauth2-proxy)

Checklist:

• My code follows the contribution guidelines of this project.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation (README.md).
• Chart.yaml includes version and appVersion (3.22.0-beta.0).
• I have added an entry to the CHANGELOG.md with the new version number and a summary of my changes (if applicable).
• I have tested my changes on a live Kubernetes cluster (optional for new chart addition).

Additional Information

Wrapper chart mirrors upstream Gatekeeper defaults; all upstream options are configurable under spec in values.yaml.

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By submitting this pull request, I confirm that my contribution is made under the terms of the project's license.