TMDM-7615: "Sub-type should not allow to define field with the same name with it's supertype"

Adds new validation rule to check for override of fields in sub types (sub type is not allowed to declare an element with same name as an inherited one).
Ensures validation rule is performed for all fields.
Adds unit test for issue.
(https://jira.talendforge.org/browse/TMDM-7615)