[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>

tests/CMakeLists.txt

@@ -369,6 +369,7 @@ cockatrice_add_test(
   ${VERSION_STRING_CPP}
   loader_local_matching_test.cpp
   LIBS
+  libcockatrice_network
   libcockatrice_settings
   INCLUDE_DIRS
   ${CMAKE_SOURCE_DIR}/cockatrice/src