feat: use go:embed to embed templates (#1756)

tools/goctl/api/gogen/middleware.tpl

@@ -0,0 +1,19 @@
+package middleware
+
+import "net/http"
+
+type {{.name}} struct {
+}
+
+func New{{.name}}() *{{.name}} {
+	return &{{.name}}{}
+}
+
+func (m *{{.name}})Handle(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		// TODO generate middleware implement function, delete after code implementation
+
+		// Passthrough to next handler if need
+		next(w, r)
+	}
+}