`format` is fixed to json

[nocollier]

Once we are happy with the way the json responses are working, we need to provide a conversion to xml.

[jpnavarro]

From slack:
John-Paul Navarro: For ESGF-1.5 can we safely drop XML search response support in esg-search (and other tools)?

Sasha Ames: Uncertain. If the major clients use that rather than .json then we may need to add the XML wrapper to the FAST_API, so there is a dependency on some application testing. We could push back on the delivery timeframe of that feature though.
those clients so far are esgpull and ESMValTool. Most Python applications prefer .json.