Consider writing an analyzer for common IHttpForwarder usage mistakes

[MihaZupan]

For example

• [ ] warn if we see an unconditional SocketsHttpHandler allocation in the same method as the SendAsync call, meaning that connections will never be pooled. We've seen such examples in several issue repros shared with us.
• [ ] (feel free to add more)

(assuming we can avoid too many false positives)