With this solution, somewhere deep down within the implementation
the knowledge about the actual result type would be encoded into
the embedded VTable within a lib::variant. At interface level,
ther will be a double dispatch based on that result type
and the desired result type, leading either to a successful
access or an error response.
Basically the mocking mechanism just switches the configuration
and then waits for the service to be accessed in order to cause acutual
instantiation of the mock service implementation. But sometimes we want
to prepare and rig the mock instance prior to the first invocation;
in such cases it can be handy just to trigger the lazy creating process
