Create CI for structurizr

This implements the requested CI for planning#37

Related to !2

@kim Do you have any wishes/requirements on d2 flags? Specifically wondering about theme and layout engine.

Functionality

It installs structurizr and d2 inside the CI. The tool versions are defined as the variables on top.

It then checks for .dsl files within the structurizr folder. It takes the basename of it, creates an outdir at build/${basename} for d2, png and mermaid. Into those, it exports the various formats. This allows having the graphs organized by their source file.

At the end we create an artifact for these build results which is stored for 4 weeks.