`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.

先从 ClientSessionGroup 及其对 ServerParameters 的处理入手，然后将其与 examples/clients/simple-auth-client/mcp_simple_auth_client/main.py 中引用的 singleton OAuth 流程进行比较。确认应如何提供 OAuth httpx.Auth provider，并添加一个使用相同流程的 ClientSessionGroup 示例。完成的标准是该 group 支持此 provider，而无需手动传递 bearer token，并且示例能够展示这一点。