Add a basic flow graphic visual representation

[ricardozanini]

What would you like to be added:
Use Mermaid to export a Workflow definition to a visual representation of the workflow.

This can be a new module to not add additional dependencies to the core package. Users who require such a feature can simply import the dependency into their project.

The module can generate a string description representation of Mermaid of a given workflow and use other tools to export it to a PNG/SVG. Optionally, we may offer both. String and image export.

Why is this needed:
The Java DSL is lacking a good visual representation since not all nodes have a YAML representation yet.