REST API endpoints for adding/removing group CA for a top-level group
## Overview
More information about the motivation in epic: https://gitlab.com/groups/gitlab-org/-/epics/10662+
## Proposal
It should be possible for a group admin (users that can change group settings) to list/add/remove group CA via REST API.
- `GET /api/v4/groups/:id/ssh_certificates` lists all certificates created for a group
- `POST /api/v4/groups/:id/ssh_certificates`
The endpoint should accept `key` (public key of CA), `title` (an arbitrary name for the key) and create `Groups::SshCertificate` [record](https://gitlab.com/gitlab-org/gitlab/-/issues/421537) on a top-level group. The `app/services/groups/ssh_certificates/create_service.rb` can be extracted from https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126741+. If the group is not a top-level group, the request is rejected (400 or 422). Let's also put this functionality behind a feature flag, so we could release it along with other functionality related to SSH certificates.
- `DELETE /api/v4/groups/:id/ssh_certificates/:ssh_certificate_id` should delete the record. A separate service class may be defined but it's not necessary since it's as simple as `certificate.destroy`
### Availability and Testing
Add new specs for new GET, POST and DELETE endpoints under `ee/spec/requests/groups`.
issue