feat: add Helm Chart support with auto-generation from Kustomize

[ballista01]

Inspired by #95 (comment) I tried to add a Helm Chart support for etcd-operator. The Chart is auto-generated from Kustomize manifests to maintain a single source of truth.

Changes

Helm Chart Generation

• make helm generates Chart from Kustomize using helmify and yq
• Includes automatic helm lint validation
• Generated files excluded from git (Chart.yaml, values.yaml, templates/, crds/)

Deployment Methods

• E2E tests now support both Kustomize and Helm
• Use DEPLOY_METHOD env var: "kustomize", "helm", or "all" (default)
• Both methods deploy functionally identical operators

Documentation

• Added helm/README.md with usage guide
• Updated main README.md with deployment options
• Included example configs: values-ha.yaml (3 replicas, leader election) and values-minimal.yaml

Testing

• Unit tests for Chart generation, validation, and values
• E2E tests pass with both deployment methods
• Tested with cert-manager v1.19.1 and etcd v3.6.5
• Verified HA setup with 3-node cluster and leader election

Architecture

• Kustomize manifests remain the source of truth
• Helm Chart auto-generated during build/test (not committed)
• CRDs installed by Helm but not deleted on uninstall
• Tools (helmify, yq) auto-install

Breaking Changes

None. Existing Kustomize deployments work unchanged.

Closes #94

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[ballista01]

@ivanvc @ahrtr Here's my attempt with Helmify. Most of it is documentation (~400 lines), tests (~700 lines), and examples (~100 lines). Core logic is just ~100 lines in Makefile. Let me know if you prefer smaller PRs (e.g. keeping only makefile changes)

[ivanvc]

I did briefly review this. I believe it should be broken down into at least separate commits. Also, the README.md appears to point to another chart.

I'll come back to this after 0.2.0.