fix(common): enforce string type in validationpipe

strictly parse the value to string to avoid nested objects being passed if the type is explicitly set to string

closes #14234

packages/common/pipes/validation.pipe.ts

@@ -206,6 +206,9 @@ export class ValidationPipe implements PipeTransform<any> {
     if (metatype === Number) {
       return +value;
     }
+    if (metatype === String) {
+      return String(value);
+    }
     return value;
   }