Add support for managing SSH certificates on a sub-group level
<!--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=470468)
</details>
<!--IssueSummary end-->
<!-- This template is a great use for issues that are feature::additions or technical tasks for larger issues.-->
### Proposal
We have a [customer](https://gitlab.zendesk.com/agent/tickets/543601) that is interested in using SSH certificates on a sub-group level.
This improves the usability of SSH certificates if the sub-group owner doesn't have owner rights to the top-level group.
<!-- Label reminders
Use the following resources to find the appropriate labels:
- Use only one tier label choosing the lowest tier this is intended for
- https://gitlab.com/gitlab-org/gitlab/-/labels
- https://about.gitlab.com/handbook/product/categories/features/
-->
issue