Allow passing a custom authentication class to httpx client

[vruello]

When instantiating Elasticsearch or AsyncElasticsearch, it is possible to pass a parameter named http_auth.

If node_class is requests and http_auth is a subclass of requests.auth.AuthBase, http_auth is passed to client_node_configs which stores it in the node config (extra["requests.session.auth"]) (https://github.com/elastic/elasticsearch-py/blob/main/elasticsearch/_sync/client/utils.py#L130).

The requests node retrieves this parameter from config and passes it to the requests session
(https://github.com/elastic/elastic-transport-python/blob/main/elastic_transport/_node/_http_requests.py#L147).

This behavior was introduced by #1944 (elasticsearch-py) and elastic/elastic-transport-python#48 (elastic-transport-python).

requests is not the only http client to support a custom authentication class. It can also be done by httpx (auth parameter of Client or AsyncClient (https://www.python-httpx.org/api/#asyncclient)).

Would you accept a PR that adds the support for http_auth when using the httpx node class?

[pquentin]

Hey @vruello, thanks for asking! We would be happy to accept such a pull request. We considered renaming http_auth to requests_http_auth to avoid confusion and then decided against it because it could instead be implemented for other HTTP clients, just as you suggest.