multi: add socks proxy flag

[ellemouton]

This PR addresses this comment suggestion to add a --proxy option to proxy http requests for price data over Tor.

It seems to me from testing that I have done than CoinCap does not allow requests over tor, but CoinDesk does. So in this PR the default backend is CoinCap if no proxy is set and is CoinDesk if a proxy is set. If both proxy and backend are specified then a validation check is done to check if the specified backend supports proxy requests.

[lightninglabs-deploy]

@ellemouton, remember to re-request review from reviewers for your latest update

[ellemouton]

!lightninglabs-deploy mute