TMDM-7626 : "Error while saving composite keys staging record"

Fix copy() issue for entity types (copy loosing key field declaration original order).
Adds unit test for issue.
(https://jira.talendforge.org/browse/TMDM-7626)
Merge remote-tracking branch 'origin/maintenance/5.6' into prototype/metadata_builder
Continue implementation (parsing of XSD simple type first working. First level elements working).