Allow calling binance API through a proxy

I see you guys use Faraday to make a request, which supports calling through a proxy. Can you expose that functionality to the user?

I think allowing it through the `Binance::Spot.new` interface is one of the ways to go, users can set it just like they set the key & secret. It'll look like this: `Binance::Spot.new(key: key, secret: secret, proxy_url: proxy_url)`

Start by locating the Faraday request setup and the Binance::Spot.new initialization described in the issue. Trace how key and secret options are stored and passed into requests, then check the existing tests around client initialization. Done means users can provide proxy_url through Binance::Spot.new and requests use it without breaking existing initialization behavior.

Written by the indexing model from the issue text.