introduce std::string constructor for ReadHandle/WriteHandle (ATLASG-1634)
That allows to instantiate the handles directly from a string. Right now if a VarHandle gets instantiated with an std::string it will create a temporary VarHandleKey which the VarHandle then references, but which will be immediately destroyed, leading to a crash when the handle is used.
This is only doing this for ReadHandle/WriteHandle, not for ReadDecorHandle/WriteDecorHandle, as those don't have that capability in the Athena implementation either and will crash there as well.