Support for generation of marker types

[ahjohannessen]

Hi, I wondered if it is possible to generate classes that extend existing (marker) traits? An example:

// General marker for all domain events
trait DomainEvent {
  def persistenceId: String
  def correlationId: UUID
  ...
}

// Specific customer events in a specific package
// Optimally it should be sealed, but that does probably 
// not work because of code gen.

trait CustomerEvent extends DomainEvent { 
  def customerId: ...
  ...
}

// Result of code generation from corresponding .proto files
case class CustomerPolicyChanged(...) extends CustomerEvent
case class CustomerStatusUpgraded(...) extends CustomerEvent
...