feat: Helmfile renders *.yaml.gotmpl in a K8s manifests/kustomization directory (#1745)

Related to #494

This feature is mostly a built-in alternative to the `incubator/raw` chart without external dependency and has
access to helmfile's own template functions and template data.

The expected use-case of this feature is to add arbitrary K8s resources to your deployment.

Unlike the original issue raised in #494 this doesn't enable you to add arbitary resources to a release. That's another story. But this would be a good foundation for that, too.

pkg/tmpl/context_tmpl.go

@@ -6,7 +6,7 @@ import (
 	"text/template"
 )
 
-func (c *Context) newTemplate() *template.Template {
+func (c *Context) CreateFuncMap() template.FuncMap {
 	aliased := template.FuncMap{}
 
 	aliases := map[string]string{
@@ -27,6 +27,12 @@ func (c *Context) newTemplate() *template.Template {
 		funcMap[name] = f
 	}
 
+	return funcMap
+}
+
+func (c *Context) newTemplate() *template.Template {
+	funcMap := c.CreateFuncMap()
+
 	tmpl := template.New("stringTemplate").Funcs(funcMap)
 	if c.preRender {
 		tmpl = tmpl.Option("missingkey=zero")