We want to alert group admins to potential additional charges when they
add more users to a group than they have purchased seats for.

Respond to the members added event by checking the usage data of the
namespace and email the user after an overage occurs by calling the
CustomersDot API.

EE: true
Changelog: added
Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/348487
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79807