Support adding expiration later on

[CodeProKid]

Should add support for adding an expiration to a registered transient after registering it once already. The use case would be: someone registers a transient without an expiration, but later on decides that it should have an expiration. Since the data shape, and how the data is stored changes slightly when a transient has an expiration, some weirdness could happen when adding the expiration after the data has been saved.

If a transient is registered with an expiration, but the data is not in the correct shape, we should just regenerate that data on the spot to be in the correct shape, so that it is correct moving forward.

Conversely, this should also work going the other way (a registered transient that has an expiration, but you want to remove it).