Add validation hook to template command execution

kyaml/fn/framework/framework.go

@@ -392,6 +392,11 @@ type Defaulter interface {
 	Default() error
 }
 
+// Validator is implemented by APIs to have Validate invoked
+type Validator interface {
+	Validate() error
+}
+
 func (tc *TemplateCommand) doPreProcess(rl *ResourceList) error {
 	// do any preprocessing
 	if tc.PreProcess != nil {
@@ -531,6 +536,12 @@ func (tc TemplateCommand) Execute(rl *ResourceList) error {
 		}
 	}
 
+	if v, ok := rl.FunctionConfig.(Validator); ok {
+		if err := v.Validate(); err != nil {
+			return err
+		}
+	}
+
 	if err := tc.doPreProcess(rl); err != nil {
 		return err
 	}