idz: Generalize msg00AD

The purpose of this request is unknown, but its message code
changed in idz2.

src/idz/userdb/handler/unknownA.ts

@@ -0,0 +1,10 @@
+import { UnknownRequestA } from "../request/unknownA";
+import { GenericResponse } from "../response/generic";
+import { Repositories } from "../repo";
+
+export function unknownA(
+  w: Repositories,
+  req: UnknownRequestA
+): GenericResponse {
+  return { type: "generic_res" };
+}