Add EmailEvents for logging in or registering during checkout

We may also want to accommodate the new settings in Commerce 2.36 that support the automatic creation of a user account on checkout completion in this issue.

Created issue for Commerce Core to create the new Event for registration during checkout: https://www.drupal.org/project/commerce/issues/3388613 📌 Create Event for registration during checkout Active

Once the Commerce Core issue has been committed, this MR will be unblocked and will add a new EmailEvent for registration during checkout.

@rszrama
Note that when the commerce_checkout.checkout_register event is fired, the order has not yet been placed. So, certain information such as [commerce_order:total_paid] will not be available yet.
This is because in extractEntityFromEvent() we return the order but not the account that was just created. But in this case, we can get the account from the order [commerce_order:uid:entity] and also use some information available for the order. If we return the account, we will not be able to access the order information.
But it looks like the purpose of this issue is to work with the account, so probably we shouldn't think about the order and just return the account in extractEntityFromEvent()?

Committed!

Automatically closed - issue fixed for 2 weeks with no activity.