[keycloak-dev] Preview and optional features

We currently have a mechanism in place to allow features to be marked as preview, experimental and disabled-by-default. Enabling/disabling these features requires restarting the server and also it is a global option. It may be better to instead have these as options on a realm. This would allow enabling/disabling such features at runtime, and it would be possible to enable through the admin console directly. It also has the benefit of allowing certain features only for certain realms. For example you may want to enable impersonation only for a specific realm, not for all realms. Same with a preview feature you may want to enable it in a test realm, not for all realms. Thoughts?