[Compiler] Support all event types

[wwerner]

Currently, all types are generated as subclasses of DomainEvent.
Other types need to be declared and used in io.vlingo.schemata.codegen.backend.java.JavaBackend#baseClassOf

[VaughnVernon]

@wwerner For now I would prefer to have standard base classes only for Command and DomainEvent. I have tried to think of good reasons for standards for the other types, such as Data, Document, and Envelope, but I can't.

See: #98 (comment)

[hurelhuyag]

This issue should be fixed by #180