'insertHandler' is a method introduced in Jetty 9, Jetty 8 should
however be supported so 'setSecurityHandler' does the same thing but
works for both 8.x and 9.x;
This adds support for using Jetty together with the Keycloak
spring-boot-adapter. Sadly the KeycloakSpringBootProperties.SecurityCollection
definition is mostly inspired by Tomcats SecurityConstraint/SecurityCollection
which doesn't provide a good fit for jettys structures.
In cases where jetty only allows one setting, we use the first value.
We only initialize KeycloakJettyServerCustomizer if jetty is used, same
applies for tomcat.
Revised configuration and extracted serverCustomizer code into
dedicated classes.
Prepared infrastructure for Undertow support.
