netxlite: support dialing with HTTP/HTTPS proxies

[bassosimone]

This issue captures the work started by @Murphy-OrangeMud in ooni/probe-cli#1162. As I mentioned in the ooni/probe-cli#1162 (review), we now have overall support for proxies but having the ability to change the proxy for each dial would allow for fine grained proxies (i.e., per domain) as well as for hopping between proxies depending on what happens (e.g., maybe we start seeing failures and use another proxy if multiple proxies, possibly per target are configured). The precondition to use this functionality is to have an HTTP/HTTPS proxy dialer.