Add mechanism for Gitlab import/export to add import only attributes
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The following discussion from !124375 should be addressed:
- [ ] @mcelicalderonG started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/124375#note_1450280280): (+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
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