Extend custom notification "Added as approver" feature to groups
## Background
In 16.7 we added a new custom notification event "Added as approver", this was discussed extensively in [Optional Merge Request notifications for approvers](https://gitlab.com/gitlab-org/gitlab/-/issues/12855) and was later added with [Added approver notification setting](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137006 "Added approver notification setting").
> This is good that individual approvers will get notified starting GitLab version 16.7 however I believe it's not easy to manage users individually in repo's rather editing a user in a group is much more streamlined solution and would be beneficial if GitLab can look into it.
## Problem to Solve
The option for a notification when `added as an approver` can only be set on a users individual global notification level OR on the project level. This means it's not possible to set the option to be notified for this for only certain groups.
## Proposal
The option to be notified when `added as an approver` should be added to the custom configuration option when configuring the notification level on a group.
### Interested Customers:
- https://gitlab.zendesk.com/agent/tickets/517208 (internal use only)
issue