Buyer Email for PayPal One Time Checkout

[tdchow]

Summary of changes

• Move userAuthenticationEmail from PayPalVaultRequest to the abstract PayPalRequest so that it's included in PayPalCheckoutRequest

Checklist

• [X] Added a changelog entry
• [X] Relevant test coverage

Authors

List GitHub usernames for everyone who contributed to this pull request.

[tdchow]

Did we validate it's working in prod too?

I'm not seeing the email prefilled in Live 😞 We'll need to reach out to the API team to take a look.