AsyncTask: Centralize setting global future synchronizer

Move the global FutureSynchronizer to Utils and use it
by default for AsyncTask (if that is in the main thread).
This way setting a synchronizer is less prone to be forgotten.
Individual uses can still override this.

Change-Id: I1a12bf4d7f4cb5be99668bf0a4797108c2fe6448
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/coreplugin/plugininstallwizard.cpp

@@ -242,7 +242,6 @@ public:
                 return SetupResult::StopWithError;
 
             async.setConcurrentCallData(checkContents, m_tempDir->path());
-            async.setFutureSynchronizer(PluginManager::futureSynchronizer());
             return SetupResult::Continue;
         };
         const auto onCheckerDone = [this](const Async<ArchiveIssue> &async) {