Feature Request: add the generated legend with "sructurizr" renderer

[jeremycohensolal-paylead]

cf this discussion: #652

it would be a good idea to add the ignored, but generated, legend in the built output when rendered with "stucturizr" exporter.

[jenspav]

Some technical details: the legend can be found in com.structurizr.export.Diagram.getLegend().getDefinition() for the structurizr-plantuml exporter. getLegend() is null for the c4-plantuml exporter since that exporter renders the legend in the diagram itself (unless c4plantuml.legend is false).