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/providers/NetworkConfigurationProvider.hpp

@@ -0,0 +1,61 @@
+#pragma once
+
+#include "common/QLogging.hpp"
+
+#include <QNetworkProxy>
+#include <websocketpp/connection.hpp>
+
+#include <string>
+
+namespace chatterino {
+
+class Env;
+
+/** This class manipulates the global network configuration (e.g. proxies). */
+class NetworkConfigurationProvider
+{
+public:
+    /** This class should never be instantiated. */
+    NetworkConfigurationProvider() = delete;
+
+    /**
+     * Applies the configuration requested from the environment variables.
+     *
+     * Currently a proxy is applied if configured.
+     */
+    static void applyFromEnv(const Env &env);
+
+    template <class C>
+    static void applyToWebSocket(
+        const std::shared_ptr<websocketpp::connection<C>> &connection)
+    {
+        const auto applicationProxy = QNetworkProxy::applicationProxy();
+        if (applicationProxy.type() != QNetworkProxy::HttpProxy)
+        {
+            return;
+        }
+        std::string url = "http://";
+        url += applicationProxy.hostName().toStdString();
+        url += ":";
+        url += std::to_string(applicationProxy.port());
+        websocketpp::lib::error_code ec;
+        connection->set_proxy(url, ec);
+        if (ec)
+        {
+            qCDebug(chatterinoNetwork)
+                << "Couldn't set websocket proxy:" << ec.value();
+            return;
+        }
+
+        connection->set_proxy_basic_auth(
+            applicationProxy.user().toStdString(),
+            applicationProxy.password().toStdString(), ec);
+        if (ec)
+        {
+            qCDebug(chatterinoNetwork)
+                << "Couldn't set websocket proxy auth:" << ec.value();
+        }
+    }
+};
+
+}  // namespace chatterino