feat: Update specification and client builder

[micha91]

• Update jinja templates for the new openapi-generator version 0.19.1
• Use a python instead of a bash script to generate the client
• Fix the following errors in the spec using the script
  • objects in Errors are marked as nullable
  • this makes patching the response coming from polarion obsolete
  • 4XX-5XX error contents are deleted from the spec and instead added to the individual error codes
  • this makes parsing the errors manually obsolete
  • In some JobSchemes there is a downloads section, which is an array. By openAPI rules arrays have to describe items, but they are missing in the spec
  • in importXUnitTestResults the body should be a file provided a s octet-stream. This should be described as string in binary format, but is for some reason described as object
• In some cases the order of arguments changed. For that reason, we now use a KW only approach when we use the auto generated client
  • This is not enforced, so please double check, if I missed it somewhere in the non auto-generated code

[micha91]

I guess everything else is generated. So after passing all checks and resolving my suggestions: LGTM.

Please mind that there are also changes in the client.py. E.g. using kw-based approaches and we now have a slightly different error handling. Could you also take a look into that?