Add customer details section to new purchase flow

Problem

Follow up from #298721 (closed)

Design issue: #322085 (closed)

Users with the following setup are unable to purchase in the new purchase flow as they do not have a last name set, which is required for the creation of a customer in the customers-dot app:

1. User does not have a last name (signed up before we split the name field) and their full name only has one word
2. User does not have an existing customers dot account
3. User is creating the subscription on .com
4. User is creating the subscription for a group (ie, using the purchase flow that does not redirect to customers-dot)

For now we are working around this by directing users without a last name to the old purchase flow, as we receive this error otherwise:

Goal

To add fields to the new purchase flow in GitLab that allow the user to submit a last name to customers-dot when they don't have one set in GitLab.