Better openapi readmes

As started in this discussion, @rigelk this MR adds the possibility to customize the READMEs generated by the openai-generator.

I modified the README.mustache for python (copied from github ). DefaultGenerator and each language's generator e.g PythonClientCodegen.java can be searched for additionalProperties.put which will contain more fields for the templates.

The MR is editable so you can edit the README as you like and see the results in https://framagit.org/peertube-test/apis/ e.g python has "THIS IS A CUSTOM README" at the beginning.