Utils: Add std::expected implementation

Adds a std::expected implementation that is compatible with >= C++11.

FilePath::fileContents and FilePath::writeFileContents as well as
FilePath::copyFile are changed to return std::expected.

A couple of macros have been added to aid in using the expected types.

An auto test was added showing how to use the library.

Change-Id: Ibe3aecfc1029a0cf13b45bf5184ff03a04a2393b
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/cmakeprojectmanager/fileapiparser.cpp

@@ -75,7 +75,7 @@ static QJsonDocument readJsonFile(const FilePath &filePath)
     qCDebug(cmakeFileApi) << "readJsonFile:" << filePath;
     QTC_ASSERT(!filePath.isEmpty(), return {});
 
-    const std::optional<QByteArray> contents = filePath.fileContents();
+    const expected_str<QByteArray> contents = filePath.fileContents();
     if (!contents)
         return {};
     const QJsonDocument doc = QJsonDocument::fromJson(*contents);