Restrict namespace API billing fields based on user permissions

What does this MR do and why?

Restrict the following namespace API billing fields based on user permissions.

seats_in_use
max_seats_used
max_seats_used_changed_at
end_date

This change is behind a feature flag. When enabled, these fields are only returned to users with billing access permissions.

References

Issue: #517976 (closed)

How to set up and validate locally

1. Create 2 users
2. As "user 1" create a group and make note of the group id.
3. Invite "user 2" to the group as a guest
4. As "user 2" fetch the namespace via API

curl -X GET 'http://localhost:3000/api/v4/namespaces/<group_id>' \
  --header 'PRIVATE-TOKEN: <user-2-api-token>'

5. Notice fields are returned

{
  "id": 108,
  "name": "first_group",
  "path": "first_group",
  "kind": "group",
  "full_path": "first_group",
  "parent_id": null,
  "avatar_url": null,
  "web_url": "http://127.0.0.1:3000/groups/first_group",
  "billable_members_count": 2,
  "seats_in_use": 2,
  "max_seats_used": 0,
  "max_seats_used_changed_at": null,
  "end_date": null
}

6. In rails console enable the feature flag

Feature.enable(:restrict_namespace_api_billing_fields)

7. Run the same curl, notice billing fields are now null

{
  "id": 108,
  "name": "first_group",
  "path": "first_group",
  "kind": "group",
  "full_path": "first_group",
  "parent_id": null,
  "avatar_url": null,
  "web_url": "http://127.0.0.1:3000/groups/first_group",
  "billable_members_count": null,
  "seats_in_use": null,
  "max_seats_used": null,
  "max_seats_used_changed_at": null,
  "end_date": null
}

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