Add code generator for OCPP 2.1

[OrangeTux]

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This PR introduces support for OCPP 2.1. The code in ocpp/v21 is generated by the newly introduced code generator.

I've created this draft to gather feedback on the generated code in ocpp/v21, not on the code generator itself. The generator requires some clean up and tests.

Unfortunately, I'm not allowed to include the JSON schemas with this PR at the moment. The OCA distributed these files to members only. I'm in touch with the OCA to see if they allow me to include the schemas in this project.

Fixes: #452