Add a skeleton subsystem for keycloak and add an operation to migrate from the keycloak subsystem to the elytron-oidc-client subsystem

We need the ability to migrate from the keycloak subsystem that used to be used with the Keycloak OIDC adapter to the elytron-oidc-client subsystem.

The keycloak subsystem itself was part of the Keycloak code base, it wasn't part of the WildFly code base.

That means that in order to provide a migrate() op ourselves, we would need to first add a skeleton subsystem for keycloak to the WildFly code base and then create the migrate() op to easily migrate to the elytron-oidc-client subsystem.