business key cannot be specified when process are started via signal/message event

At present there is no way to start a process passing down the business key for process instances causing this one to be always the process instance id.

We can specify several parts in this matter

1. transport : jms, kafka or rest (for sending a signal)
2. headers in the tranport layer: would have something lakie correlation key header

For that happening we need to move the Signal from simple string to an object at least at the service layer of the engine so we can add metainformation to the signal

Old methods would be replace calling the the new ones just creating a Signal.of("my signal", Collections.singletonMap("correlationKey", "mycorrelationKey"))