Add authorization to the group's members API endpoint

What does this MR do and why?

This change adds a security check to the member creation API endpoint for projects and groups. Before allowing someone to add new members to a project or group, the system now verifies that the person making the request has administrator permissions for that specific project or group. This prevents unauthorized users from adding members to resources to which they shouldn't have access.

References

• Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/546378+

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