Add a mitmMutateRequest hook and add ConnectAction to ProxyCtx

[harold-stripe]

This PR adds 2 things

• A hook that enables the caller code to mutate the request via a hook. I've tested this working on smokescreen and added a test.
• Setting ConnectAction in ProxyCtx. proxy.OnRequest().DoFunc is called on the MITM request (the outbound one from goproxy to the remote host) and adding this variable makes it easy to know if target is a "traditional" HTTP Proxy of if the MITM request.

[CLAassistant]

All committers have signed the CLA.

[cds2-stripe]

LGTM!