Trigger an email when seat overage occurs

What does this MR do and why?

Addresses part of: https://gitlab.com/gitlab-org/gitlab/-/issues/348487

⚠️ This is dependent on https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/4436

Respond to MembersAddedEvent to check if there has been a seat overage. If there has, trigger an API request to CustomersDot to email the group owners about the overage.

How to set up and validate locally

With CustomersDot running locally:

1. Create a new group on GitLab.com
2. Purchase a new subscription with 1 seat
3. Add a new user to the group
4. Observe the job has been triggered in log/sidekiq.log
5. View the emails in http://localhost:5000/letter_opener/

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.