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.cpp

@@ -0,0 +1,76 @@
+#include "providers/NetworkConfigurationProvider.hpp"
+
+#include "common/Env.hpp"
+#include "common/QLogging.hpp"
+
+#include <QNetworkProxy>
+#include <QSslConfiguration>
+#include <QUrl>
+
+namespace {
+/**
+ * Creates a QNetworkProxy from a given URL.
+ *
+ * Creates an HTTP proxy by default, a Socks5 will be created if the scheme is 'socks5'.
+ */
+QNetworkProxy createProxyFromUrl(const QUrl &url)
+{
+    QNetworkProxy proxy;
+    proxy.setHostName(url.host(QUrl::FullyEncoded));
+    proxy.setUser(url.userName(QUrl::FullyEncoded));
+    proxy.setPassword(url.password(QUrl::FullyEncoded));
+    proxy.setPort(url.port(1080));
+
+    if (url.scheme().compare(QStringLiteral("socks5"), Qt::CaseInsensitive) ==
+        0)
+    {
+        proxy.setType(QNetworkProxy::Socks5Proxy);
+    }
+    else
+    {
+        proxy.setType(QNetworkProxy::HttpProxy);
+        if (!proxy.user().isEmpty() || !proxy.password().isEmpty())
+        {
+            // for some reason, Qt doesn't set the Proxy-Authorization header
+            const auto auth = proxy.user() + ":" + proxy.password();
+            const auto base64 = auth.toUtf8().toBase64();
+            proxy.setRawHeader("Proxy-Authorization",
+                               QByteArray("Basic ").append(base64));
+        }
+    }
+
+    return proxy;
+}
+
+/**
+ * Attempts to apply the proxy specified by `url` as the application proxy.
+ */
+void applyProxy(const QString &url)
+{
+    auto proxyUrl = QUrl(url);
+    if (!proxyUrl.isValid() || proxyUrl.isEmpty())
+    {
+        qCDebug(chatterinoNetwork)
+            << "Invalid or empty proxy url: " << proxyUrl;
+        return;
+    }
+
+    const auto proxy = createProxyFromUrl(proxyUrl);
+
+    QNetworkProxy::setApplicationProxy(proxy);
+    qCDebug(chatterinoNetwork) << "Set application proxy to" << proxy;
+}
+
+}  // namespace
+
+namespace chatterino {
+
+void NetworkConfigurationProvider::applyFromEnv(const Env &env)
+{
+    if (env.proxyUrl)
+    {
+        applyProxy(env.proxyUrl.get());
+    }
+}
+
+}  // namespace chatterino