refactor: make Variant constructor explicit

[sangelovic]

This makes the library more robust and prone to user's errors when the user writes an extension for their custom type. In case they forget to implement a serialization function for that type and yet insert an object of that type into sdbus::Message, the current behavior is that, surprisingly, the library masks the error as it resolves the call to the Variant overload, because Variant provides an implicit template converting constructor, so the library tries to construct first the Variant object from the object of custom type, and then inserting into the message that Variant object. Variant constructor serializes the underlying object into its internal message object, which resolves to the same message insertion overload, creating an infinite recursion and ultimately the stack overflow. This is undesired and plain wrong. Marking this Variant converting constructor solves these problems, plus in overall it makes the code a little safer and more verbose. With explicit Variant constructor, when the user forgets to implement a serialization function for their type, the call of such function will fail with an expressive compilation error, and will produce no undesired, surprising results.