Proxy can not be set for AsyncOAuth2Client

[sebastian-heinz]

Describe the bug

AsyncOAuth2Client accept kwargs that are passed to the httpx.AsyncClient.

httpx.AsyncClient accept proxy as a kwarg.
however:
client_kwargs = self._extract_session_request_params(kwargs)

drops anything that is not in here:
HTTPX_CLIENT_KWARGS = [
'headers', 'cookies', 'verify', 'cert', 'http1', 'http2',
'proxies', 'timeout', 'follow_redirects', 'limits', 'max_redirects',
'event_hooks', 'base_url', 'transport', 'app', 'trust_env',
]

proxies afaik is used in requests or other libraries to provide a dict of protocol/host, however httpx client seems to only expect proxy kwarg, which is a string with protocol://host format.

Expected behavior

passing proxy="http://host.tld:8080" as kwarg to AsyncOAuth2Client should be passed to the underlying httpx client

Environment:

• OS: osx
• Python Version: 3.12.7
• Authlib Version: 1.3.2

[lepture]

It seems httpx has changed its parameter.