Update Checkout creation to use merchant_code instead of pay_to_email

[lordofweb]

The current SumUp API documentation for creating checkouts (https://developer.sumup.com/api/checkouts/create#checkouts-create) recommends using merchant_code instead of pay_to_email. However, the current SDK still uses pay_to_email.

According to the documentation:
"Email address of the registered user (merchant) to whom the payment is made. It is highly recommended to use merchant_code instead of pay_to_email."

This issue proposes updating the SDK to align with the current API recommendations by replacing pay_to_email with merchant_code in the Checkout creation process.

This change would involve:

1. Updating the create method signature in the Checkouts class
2. Modifying the payload construction to use merchant_code
3. Updating relevant documentation and tests

A pull request with these changes will be submitted for review.