Provide access to unwrapped value for ConstraintViolation on container method argument or return value

[rstoyanchev]

For method validation on arguments or return values that are containers, it would be useful to have access to the unwrapped value. For lists, arrays, and maps it is possible to get the containerIndex or containerKey from the next Path.Node and use that to get the value, but there is nothing for Iterable and other containers.

ConstraintViolation exposes part of the object graph through rootBean and leafBean. Perhaps Path.Node could provide access to the unwrapped value associated with that part of the property path, but there may be other ways.