Support validation non PersistentEntity types

[f-cramer]

This change is required to perform validation on converted fields of JPA entities. Their types might not be managed types and as such no JPAPersistentEntity instances are created for them. Before these changes this would result in a NotReadablePropertyException every time there is a validation error in one of those fields.

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[f-cramer]

any chance to get this reviewed before the next release on friday?

[f-cramer]

Is there anything more I can do to get this ready to be merged?