support h2 for http proxy client connector

[GlenDC]

the current client http proxy client connector is hardcoded for h1. This is fine and dandy for most use cases, but in case you are dealing with a h2 only server this won't work.

https://github.com/plabayo/rama/blob/0abc1495acf91bd6a1eda72bf1662eee424a9db3/rama-http-backend/src/client/proxy/layer/proxy_connector/connector.rs

Goal would be to make it auto by default. but that one can also set it to h1 or h2.
The auto mode would try to use the request protocol version and falling back to h1 if it couldn't be extracted.