[Network] Add a configured QNetworkAccessManager factory (#7628)

* [Network] Add a configured QNetworkAccessManager factory

Every HTTP caller built its own QNetworkAccessManager, so the system proxy
and a transfer timeout were easy to omit. Add NetworkManagerFactory::create()
as the single place that applies the process-wide proxy configuration and a
transfer timeout, and use it at all 14 construction sites.

Per-site settings such as disk caches and manual redirect policies stay with
their callers.

* [Tests] Link libcockatrice_network into the tests that compile its headers

release_channel.cpp and card_picture_loader_worker.cpp now include
libcockatrice/network/network_manager.h, but the test targets that compile
them only linked libcockatrice_settings, so the include directory never
reached them and every platform failed with "file not found":
loader_local_matching_test and picture_loader_benchmark_test.

libcockatrice_network is an INTERFACE library, so linking it costs these
targets nothing but hands over the include path.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/interface/widgets/server/handle_public_servers.cpp

@@ -13,13 +13,14 @@
 #include <QUrl>
 #include <QVariant>
 #include <QVariantMap>
+#include <libcockatrice/network/network_manager.h>
 #include <libcockatrice/settings/servers_settings.h>
 #include <qlogging.h>
 
 #define PUBLIC_SERVERS_JSON "https://cockatrice.github.io/public-servers.json"
 
 HandlePublicServers::HandlePublicServers(QObject *parent)
-    : QObject(parent), nam(new QNetworkAccessManager(this)), reply(nullptr)
+    : QObject(parent), nam(NetworkManagerFactory::create(this)), reply(nullptr)
 {
 }