CODEOWNERS optional groups should display group members in MR approvals
### Problem to solve
Currently, an optional approval group will display all possible approvers, however an optional `CODEOWNERS` approval group will not show any. This feels inconsistent and it would be useful to show members of the optional group
### Proposal
Display all (public) members of approval groups.
#### Example screenshot
In this example, the `Reviewers` is an optional `CODEOWNERS` group containing the same group referenced under `Default` (`@gitlab-org/secure/static-analysis-be`), however only `Default is shown:

- `CODEOWNERS` File: https://gitlab.com/gitlab-org/security-products/analyzers/bandit/-/blob/4196e5d4fb14fc746fa24ebb948cffee32998013/.gitlab/CODEOWNERS
- MR: https://gitlab.com/gitlab-org/security-products/analyzers/bandit/-/merge_requests/75
### User experience goal
- More consistency in MR approvals
- Easier discovery of expected approvers
### Permissions and Security
Matching other approval groups, only public members should be shown. If `@groupA` is a private group, show none. If `@groupB` is a public group, show each member.
### Documentation
See the Feature Change Documentation Workflow https://docs.gitlab.com/ee/development/documentation/workflow.html#for-a-product-change
* Add all known Documentation Requirements in this section. See https://docs.gitlab.com/ee/development/documentation/workflow.html
* If this feature requires changing permissions, update the permissions document. See https://docs.gitlab.com/ee/user/permissions.html
### Availability & Testing
This section needs to be retained and filled in during the workflow planning breakdown phase of this feature proposal, if not earlier.
What risks does this change pose to our availability? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing?
Please list the test areas (unit, integration and end-to-end) that needs to be added or updated to ensure that this feature will work as intended. Please use the list below as guidance.
* Unit test changes
* Integration test changes
* End-to-end test change
See the test engineering planning process and reach out to your counterpart Software Engineer in Test for assistance: https://about.gitlab.com/handbook/engineering/quality/test-engineering/#test-planning
### What is the type of buyer?
What is the buyer persona for this feature? See https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/buyer-persona/
In which enterprise tier should this feature go? See https://about.gitlab.com/handbook/product/pricing/#three-tiers
issue