Nexus Repository - Arbitrary Class Instantiation via Unsafe Realm Configuration

An account holding the nexus:settings:update permission in Nexus Repository 3 (or the equivalent nexus:settings permission in the legacy Nexus Repository 2) could submit arbitrary values as realm identifiers through an internal configuration API that did not validate them against the set of registered realms. Because unrecognized entries were persisted and re-evaluated on every realm load via a legacy code path, this could result in unintended code executing inside the Nexus Repository process, and in some cases a persistent authentication lockout that was not visible through the administrative UI.