Refactor DVLM Validator extension point

[andriikovalov-dlr]

• Currently DVLM Validator extension point expects a sequence of validators which we never use and it doesn't make sense since we can (and do) specify every validator in a separate extension.

• It would be good to add an optional field to the extension point that would indicate if a validator is concept-specific or not

This would need a change in plugin.xml of all concepts, and in the generator.