Create Externalizers, @Immutable and general session replication/failover quickstart

Demonstrate the marshalling mechanism via implementations of org.wildfly.clustering.marshalling.Externalizer in conjunction with general session replication.

To demonstrate externalizers, we can demonstrate with non-serializable objects

For @ Immutable we can take 2 classes, one is immutable, one not, we can demonstrate getting attribute and mutating object vs get attribute, mutate, subsequent requests won't see mutation on failover – set attribute must be used.