Add SubscriptionMailer for seat overages

What does this MR do and why?

Addresses: #349803 (closed)

Add a mailer that can be triggered when the user adds a member to a group, and goes over their purchased seat capacity. This MR will be followed up in https://gitlab.com/gitlab-org/gitlab/-/issues/348487 which will actually trigger the sending of the email.

Screenshots or screen recordings

How to set up and validate locally

This can be previewed using the mailer preview at http://localhost:3000/rails/mailers/gitlab_subscriptions/seat_usage_mailer/exceeded_purchased_seats.

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.