Importers: standardise import file validations
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=452230)
</details>
<!--IssueSummary end-->
Create a reusable component for importers which can perform a consistent set of file validations on import archives (and other files?).
- Archive size
- Archive type
- Archive content
issue