HV-819 Adding support for unwrapping values prior to validation

Hey, this is the PR for unwrapping values prior to validation via a new annotation `@UnwrapValidatedValue`. I'm still about to polish some details and add documentation but am keen on some feedback on the general direction.

Also I'd be very happy about alternative ideas for the type and package names; maybe the SPI interface should be called `(Validation)ValueHandler`, which makes the concept a bit more open for other means of value pre-processing besides plain unwrapping?