Preserve kwargs for json.dumps()

[fdemmer]

Pydantics' json() method forwards all unused kwargs e.g. indent to json.dumps(). However, base.extract_export_kwargs() only considers arguments for pydantic and drops others.

My usecase is having the data loaded from a Django JSONField nicely indented when shown in a form; so at least passing on indent would be useful for me.

The suggested PR allows the kwargs used to "format" the output of json.dumps(): indent, separators and sort_keys

Please let me know if I missed a way to do that without the suggested change or any other comments!

[surenkov]

Hi @fdemmer, sorry for such a long response, been 🔥 busy on my main project;
and thank you for your contribution!

I updated your PR with extra json.dump(...) kwargs which may be still reasonable for pydantic models.