Adding support for openAPI specifications using $ref

[SilviaAmAm]

When the zds_client creates the headers for a request, it filters the header parameters as follows (https://github.com/VNG-Realisatie/gemma-zds-client/blob/master/zds_client/client.py#L34):

    def filter_header_params(params: list):
        return [
            param for param in params if param.get("in") == "header" and param.get("required")
        ]

If the parameters in the API specification are specified with references (https://swagger.io/docs/specification/using-ref/), the parameters in and required will not be in the params dictionary, and an error will be raised.

The client needs to be able to deal with:

• Local references

• Remote references

• URL references