Add HTTP & SOCKS5 proxy support (#4321)

This can be configured using the `CHATTERINO2_PROXY_URL` environment variable.
The behaviour is similar to curl's CURLOPT_PROXY

src/common/Env.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <boost/optional.hpp>
 #include <QString>
 
 namespace chatterino {
@@ -16,6 +17,7 @@ public:
     const QString twitchServerHost;
     const uint16_t twitchServerPort;
     const bool twitchServerSecure;
+    const boost::optional<QString> proxyUrl;
 };
 
 }  // namespace chatterino