Checkmk Werk 12958: add validation for custom tags to REST API

ID: 12958 Title: add validation for custom tags to REST API Component: Core & setup Level: 1 Class: Bug fix Version: 2.1.0i1 Previously, host attributes which were custom defined could not be used in the REST API due to a missing validation step. This Werk fixes this. Custom attributes can now be used in the REST API as soon as they are created (e.g. using the Setup GUI).