Allow the user to pass in an `httpx_client_factory` to be used when creating the `streamablehttp_client`

[calvinbrown085]

Describe the solution you'd like
StreamableHTTPConnectionParams() Allow the user to pass in an httpx_client_factory to be used when creating the streamablehttp_client

We have some middlewares around clients that allow us to do tracing / metrics / etc. It'd be nice to be able to pass our own client in here.

[m-breitbach]

This together with httpx Event Hooks would provide a solution to #2221, I think.