Extend validation api

[kostobog]

@blcham

• Support groups (validation constraint scopes, e.g. validations constraints when creating a new entity or updating an existing entity might differ)
• validators can be extended, e.g. NamedEntityValidator is extended by FaultEventValidator introducing validation constraints specific to FaultEvent entities.
• supports uri already exists constraint for new entities as suggested in comment in PR Document one REST endpoint with springdoc #82 #150

[blcham]

LGTM