feat: add helmfile template --validate (#1135)

This adds the ability for helmfile to call `helm template --validate` introduced in helm 3.

fixes #1105

pkg/app/app.go

@@ -1114,7 +1114,7 @@ func (a *App) template(r *Run, c TemplateConfigProvider) (bool, []error) {
 			opts := &state.TemplateOpts{
 				Set: c.Set(),
 			}
-			return subst.TemplateReleases(helm, c.OutputDir(), c.Values(), args, c.Concurrency(), opts)
+			return subst.TemplateReleases(helm, c.OutputDir(), c.Values(), args, c.Concurrency(), c.Validate(), opts)
 		}))
 
 		if templateErrs != nil && len(templateErrs) > 0 {

pkg/app/app_test.go

@@ -1877,6 +1877,10 @@ func (c configImpl) Args() string {
 	return "some args"
 }
 
+func (c configImpl) Validate() bool {
+	return true
+}
+
 func (c configImpl) SkipDeps() bool {
 	return true
 }
@@ -1894,6 +1898,7 @@ type applyConfig struct {
 	values            []string
 	retainValuesFiles bool
 	set               []string
+	validate          bool
 	skipDeps          bool
 	suppressSecrets   bool
 	suppressDiff      bool
@@ -1917,6 +1922,10 @@ func (a applyConfig) Set() []string {
 	return a.set
 }
 
+func (a applyConfig) Validate() bool {
+	return a.validate
+}
+
 func (a applyConfig) SkipDeps() bool {
 	return a.skipDeps
 }

pkg/app/config.go

@@ -125,6 +125,7 @@ type TemplateConfigProvider interface {
 
 	Values() []string
 	Set() []string
+	Validate() bool
 	SkipDeps() bool
 	OutputDir() string