feat: add Helm 4 support while maintaining Helm 3 compatibility (#2262)

This commit adds comprehensive support for Helm 4 while maintaining
full backward compatibility with Helm 3. The implementation includes:

- Updated helm version detection to support both Helm 3 and Helm 4
- Added HELMFILE_HELM4 environment variable to control Helm version
- Modified helm execution paths to handle version-specific binaries
- Updated helm plugin installation to support split architecture

- Helm 4: Uses split plugin architecture (3 separate .tgz files)
  - helm-secrets.tgz
  - helm-secrets-getter.tgz
  - helm-secrets-post-renderer.tgz
- Helm 3: Continues using single plugin installation
- Updated Dockerfiles, CI workflows, and core installation code

- Helm 4 requires post-renderers to be plugins, not executable scripts
- Created Helm plugin structure for integration tests
- Updated helmfile.yaml templates to dynamically select renderer type
- Added test plugins: add-cm, add-cm1, add-cm2

- Updated integration tests for Helm 3/4 compatibility
- Created Helm 4 variant expected output files
- Fixed test determinism issues (repo cleanup between iterations)
- Added version-specific output filtering for warnings/messages

- Updated workflows to test both Helm 3 and Helm 4
- Matrix testing across Helm versions
- Updated helm-diff to v3.14.0 for compatibility

- Updated README and docs with Helm 4 information
- Added migration guidance
- Updated version requirements

All changes are backward compatible - existing Helm 3 users will
see no behavior changes.



fix: update Helm 4 lint expected output to match filtered output

The grep filter removes the semver warning, so the expected output
should not include it. Updated lint-helm4 files to match the filtered
output (warning removed, no extra blank line).

Signed-off-by: Aditya Menon <amenon@canarytechnologies.com>

examples/charts/argocd-helmfile-deployment/argocd.dockerfile

@@ -22,6 +22,6 @@ RUN wget https://releases.hashicorp.com/vault/1.5.0/vault_1.5.0_linux_amd64.zip
 USER argocd
 
 # Install helm-secrets plugin (as argocd user)
-RUN helm plugin install https://github.com/jkroepke/helm-secrets --version v3.6.0
+RUN helm plugin install https://github.com/jkroepke/helm-secrets --version v3.6.0 --verify=false
 
 ENV HELM_PLUGINS="/home/argocd/.local/share/helm/plugins/"

examples/charts/argocd-helmfile-deployment/helmfile.yaml

@@ -22,7 +22,7 @@ commonLabels:
 releases:
   - name: argocd                           # name of this release
     namespace: argocd                      # target namespace
-    createNamespace: true                  # helm 3.2+ automatically create release namespace (default true)
+    createNamespace: true                  # automatically create release namespace (default true)
     labels:                                # Arbitrary key value pairs for filtering releases
       env: prod
     chart: "."                             # the chart being installed to create this release, referenced by `repository/chart` syntax