SecureAdminHandler no longer pulls collection name for create correctly

SENTRY-805 pulls the core name from NAME and tries to get the collection name the standard way, i.e. by looking at the cloud descriptor.

But this doesn't work if the core has not been created yet.

Instead, we should build the core descriptor in the same way as the CoreAdminHandler, see https://github.com/apache/lucene-solr/blob/857bd14169db4683c29f4c8710ff9b96f5e9fe19/solr/core/src/java/org/apache/solr/handler/admin/CoreAdminHandler.java#L551