Add customer object to checkout API

[NatElkins]

Summary

Replace individual customer fields with a unified customer object that supports standard fields plus custom fields.

Changes

Before:

{
  customerName: "John",
  customerEmail: "john@example.com",
  customerExternalId: "user_123",
  requireCustomerFields: { customerName: true, customerEmail: true }
}

After:

{
  customer: {
    name: "John",
    email: "john@example.com",
    externalId: "user_123",
    company: "Acme",        // custom field
    plan: "pro"             // custom field
  },
  requireCustomerData: ["email", "name", "company"]
}

Why

• Flat structure is simpler and more flexible
• Custom fields are first-class citizens (catchall accepts any string field)
• requireCustomerData works with both standard and custom fields
• Empty strings are transformed to undefined for graceful client handling