Cancel completion callback

[joostfunkekupper]

Is your feature request related to a problem? Please describe.
It would be nice if we could handle the difference between a successful and cancel callback. At the moment there is only a single completion callback, which get's ignored if you have the updateType set to .hard.

Even so, when the SafariViewController is cancelled, that completion callback is not being used anyway, so you get no feedback.

Describe the solution you'd like
Have a clear successCallback and dismissCallback which can be used to know if the user accepted or declined the update.

Describe alternatives you've considered
Unless I've missed something, I haven't considered any alternatives.

Additional context
None.