Add support for httpx authentication class

[vruello]

This PR adds support for passing a custom authentication class to the httpx client (https://github.com/elastic/elasticsearch-py/issues/2933) with minimal changes. It is heavily inspired by the existing implementation for the requests client. Internally, it uses the already existing requests.session.auth extra parameter.

See the corresponding PR in elasticsearch-py : https://github.com/elastic/elasticsearch-py/pull/2953