Follow-up to-dos around value extractor implementation

BV API

• Move SPI to BV (also rename UnwrapValidatedValue to the name suggested in 's proposal

Implementation

• Implicit unwrapping, e.g. @Min(1) IntegerProperty myInt; currently an explicit @UnwrapValidatedValue is required

• Move retrieval of extractors for constraints (not cascaded validation) to metadata retrieval time and cache them

• Implement @ConstraintApplyTo

• Raise error if no extractor is found during cascaded validation

• Remove value unwrappers from reference docs