[Server] Use QgsBlockingNetworkRequest to download URL

1. Deduplicate code
2. We have seen crashes where we can observe the following pattern at
   the top of the stack trace. This is always happening nested inside
   another event loop somewhere, where the lower part of the stack,
   which is omitted here `[...]` can be any other `processEvents()` call
   or local `QEventLoop`, which is likely caused by interaction of the
   local event loop for downloading here with other event loops, signals
   and timers (only approximal understanding on the exact reasons from
   my side).

```
_ZNK14QMessageLogger5fatalEPKcz: /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
_ZN28QgsServerParameterDefinition10raiseErrorERK7QString: /usr/lib/libqgis_server.so.3.22.14
_ZN28QgsServerParameterDefinition10raiseErrorERK7QString: /usr/lib/libqgis_server.so.3.22.14
_ZN7QObject5eventEP6QEvent: /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
[symbol missing]: /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
[symbol missing]: /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
[symbol missing]: /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
_ZN9QMetaType4typeERK10QByteArray: /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
[symbol missing]: /usr/lib/x86_64-linux-gnu/libQt5Network.so.5
_ZN9QMetaType8typeInfoEi: /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
_ZN14QgsApplication6notifyEP7QObjectP6QEvent: /usr/lib/libqgis_core.so.3.22.14
_ZN16QCoreApplication15notifyInternal2EP7QObjectP6QEvent: /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
_ZN23QCoreApplicationPrivate16sendPostedEventsEP7QObjectiP11QThreadData: /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
_ZN27QEventDispatcherGlibPrivate31runTimersOnceWithNormalPriorityEv: /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
[...]
```

src/server/qgsserverparameters.cpp

@@ -15,9 +15,9 @@
  *                                                                         *
  ***************************************************************************/
 
+#include "qgsblockingnetworkrequest.h"
 #include "qgsserverparameters.h"
 #include "qgsserverexception.h"
-#include "qgsnetworkcontentfetcher.h"
 #include "qgsmessagelog.h"
 #include "qgsvariantutils.h"
 #include <QObject>
@@ -332,65 +332,27 @@ QString QgsServerParameterDefinition::loadUrl( bool &ok ) const
     return QString();
   }
 
-  // fetching content
-  QgsNetworkContentFetcher fetcher;
-  QEventLoop loop;
-  QObject::connect( &fetcher, &QgsNetworkContentFetcher::finished, &loop, &QEventLoop::quit );
-
-  QgsMessageLog::logMessage(
-    QObject::tr( "Request started [url: %1]" ).arg( url.toString() ),
-    QStringLiteral( "Server" ) );
   QNetworkRequest request( url );
   request.setAttribute( QNetworkRequest::CacheLoadControlAttribute, QNetworkRequest::PreferCache );
   request.setAttribute( QNetworkRequest::CacheSaveControlAttribute, true );
-  fetcher.fetchContent( request );
-
-  //wait until content fetched
-  loop.exec( QEventLoop::ExcludeUserInputEvents );
-
-  QNetworkReply *reply = fetcher.reply();
-  if ( !reply )
-  {
-    ok = false;
-    QgsMessageLog::logMessage(
-      QObject::tr( "Request failed [error: no reply - url: %1]" ).arg( url.toString() ),
-      QStringLiteral( "Server" ) );
-    return QString();
-  }
 
-  const QVariant status = reply->attribute( QNetworkRequest::HttpStatusCodeAttribute );
-  if ( status.isValid() && status.toInt() >= 400 )
-  {
-    ok = false;
-    if ( reply->error() != QNetworkReply::NoError )
-    {
-      QgsMessageLog::logMessage(
-        QObject::tr( "Request failed [error: %1 - url: %2]" ).arg( reply->errorString(), reply->url().toString() ),
-        QStringLiteral( "Server" ) );
-    }
-    const QVariant phrase = reply->attribute( QNetworkRequest::HttpReasonPhraseAttribute );
-    QgsMessageLog::logMessage(
-      QObject::tr( "Request error [status: %1 - reason phrase: %2] for %3" ).arg( status.toInt() ).arg( phrase.toString(), reply->url().toString() ),
-      QStringLiteral( "Server" ) );
-    return QString();
-  }
+  // fetching content
+  QgsBlockingNetworkRequest newReq;
+  const QgsBlockingNetworkRequest::ErrorCode errorCode = newReq.get( request, false );
 
-  if ( reply->error() != QNetworkReply::NoError )
+  if ( errorCode != QgsBlockingNetworkRequest::NoError )
   {
     ok = false;
     QgsMessageLog::logMessage(
-      QObject::tr( "Request failed [error: %1 - url: %2]" ).arg( reply->errorString(), reply->url().toString() ),
+      QObject::tr( "Request failed [error: %1 - url: %2]" ).arg( newReq.errorMessage(), url.toString() ),
       QStringLiteral( "Server" ) );
     return QString();
   }
 
-  QgsMessageLog::logMessage(
-    QObject::tr( "Request finished [url: %1]" ).arg( url.toString() ),
-    QStringLiteral( "Server" ) );
+  QgsNetworkReplyContent reply = newReq.reply();
 
-  QString content = fetcher.contentAsString();
-  ok = ( !content.isEmpty() );
-  return content;
+  ok = !reply.content().isEmpty();
+  return reply.content();
 }
 
 QUrl QgsServerParameterDefinition::toUrl( bool &ok ) const