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

tools/goctl/api/new/api.tpl

@@ -0,0 +1,12 @@
+type Request {
+  Name string ` + "`" + `path:"name,options=you|me"` + "`" + `
+}
+
+type Response {
+  Message string ` + "`" + `json:"message"` + "`" + `
+}
+
+service {{.name}}-api {
+  @handler {{.handler}}Handler
+  get /from/:name(Request) returns (Response)
+}