Prepare app store for delivering what's new information

In order to add support for "What's new" information for apps (https://github.com/nextcloud/server/issues/9165), we would like to extend the appstore with

* [ ] an XML schema to provide that information (essentially https://github.com/nextcloud/changelog_server/blob/master/schema.xsd)
* [ ] the ability to provide this information in the requested language alongside other meta data in the corresponding api call(s)