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/gitlab/gitlabparameters.cpp

@@ -129,7 +129,7 @@ static QList<GitLabServer> readTokensFile(const Utils::FilePath &filePath)
 {
     if (!filePath.exists())
         return {};
-    std::optional<QByteArray> contents = filePath.fileContents();
+    const Utils::expected_str<QByteArray> contents = filePath.fileContents();
     if (!contents)
         return {};
     const QByteArray content = *contents;