build: justfile for the build process

[ChristianBeilschmidt]

Contents:

1. Update generated clients to output of new version of openapitech-generator.
   • Only subtle changes in almost all files, but a lot of touches.
2. Use a justfile instead of a mixture of containers, python files and separated bash commands.
   • Use @openapitools/openapi-generator-cli directly instead of patched generator container
   • Use justfiles instead of generate.py and update-config.py.
   • Post-pocessing is now done after the builder script, because not every builder supports it internally.
   • yaml files in .generation/config allows using the batch command for the generator instead of passing everything in command line args. These are auto-generated in just generate-configs or just update-config, so versions match.

Addtional questions:

gäbe es irgenwie sinn den generierten code von dem postprocessing etc. zu trennen? den generierten code kann man ja eh nicht reviewen aber die eigentlich manuell gemachten Änderungen gehen komplett unter.

I guess we don't want to store the code twice without changes and with changes.