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>

pkg/helmexec/helmexec.go

@@ -1,6 +1,6 @@
 package helmexec
 
-import "helm.sh/helm/v3/pkg/chart"
+import chart "helm.sh/helm/v4/pkg/chart/v2"
 
 // Version represents the version of helm
 type Version struct {
@@ -34,6 +34,7 @@ type Interface interface {
 	List(context HelmContext, filter string, flags ...string) (string, error)
 	DecryptSecret(context HelmContext, name string, flags ...string) (string, error)
 	IsHelm3() bool
+	IsHelm4() bool
 	GetVersion() Version
 	IsVersionAtLeast(versionStr string) bool
 	ShowChart(chart string) (chart.Metadata, error)
@@ -42,4 +43,5 @@ type Interface interface {
 type DependencyUpdater interface {
 	UpdateDeps(chart string) error
 	IsHelm3() bool
+	IsHelm4() bool
 }