Add endpoint to export detailed data about user permissions

What does this MR do and why?

It adds an endpoint to export detailed data about all group and its subgroups member permission. The export itself is implemented in Export detailed membership data (!157296 - merged).

Follow-ups:

• rollout issue

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

CSV export example

flightjs_group_memberships_2024-08-30.csv

How to set up and validate locally

Enable the FF: Feature.enable(:members_permissions_detailed_export)

Graphql query to run

mutation groupMembersExport
 {
  groupMembersExport(input: {
    groupId: "gid://gitlab/Group/33"
  }) {
    errors
    message
  }
}

Then go to http://gdk.test:3000/rails/letter_opener/ to check if you received an email with generated export.

Related to #460477 (closed)