`auth` specification in `ClientSessionGroup`

Description

It would be nice for the ClientSessionGroup to be OAuth compatible by allowing us to specify the OAuth httpx.Auth provider when passing in the ServerParameters rather than passing the bearer tokens manually via the headers field. This will allow us to do the OAuth token exchange flow straight from the ClientSessionGroup.

References

Here is the singleton ClientSession example of an OAuth flow: https://github.com/modelcontextprotocol/python-sdk/blob/main/examples/clients/simple-auth-client/mcp_simple_auth_client/main.py

It would be nice to have an example of a ClientSessionGroup using that same flow.

先從 ClientSessionGroup 及其對 ServerParameters 的處理著手，然後將其與 examples/clients/simple-auth-client/mcp_simple_auth_client/main.py 中參照的 singleton OAuth 流程進行比較。確認應如何提供 OAuth httpx.Auth provider，並新增一個使用相同流程的 ClientSessionGroup 範例。完成的標準是該 group 支援此 provider，而不需要手動傳遞 bearer token，且範例能夠展示這一點。