Add mechanism for Gitlab import/export to add import only attributes

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The following discussion from !124375 (merged) should be addressed:

• @mcelicalderonG started a discussion: (+5 comments)

There should be a way to allow importing attributes that we no longer export. This way import/export can be backwards compatible when removing attributes from objects. For this particular scenario, we are removing the issues.issue_type column, so we would like to stop exporting it, but still allow older exports that contain the attribute to be imported properly