HV-828 Introducing ConstraintDefinitionContributor to allow for a plugga...

...ble discovery of additional constraint validator implementations.

* Added the suggested builder pattern
* Aligned names/packages to refer to constraint definitions
Interesting, I would have expected that the property can be given several times, each with one contributor (similar to e.g. <class> in persistence.xml). But apparently we use the comma-separated style for value handlers already.