Avoid unnecessarily use of reflection for invoking methods of class KeycloakSecurityContext

[gunterze]

Because class KeycloakSecurityContext will not be loaded by the VM before the first instance of the class is created, you may use member variable

    private final KeycloakSecurityContext ksc;

in class KeycloakContext in module dcm4chee-arc-keycloak without forcing that module org.keycloak.keycloak-core containing class KeycloakSecurityContext must be on the CLASSPATH, when running the unsecured version of the archive.