PayPalWebCheckout HandleReturnURL for App Switch

[KunJeongPark]

Reason for changes

• Add handleReturnURL function to handle url back from app switch

Summary of changes

• Added handleReturnURL to PayPalWebCheckout to handle app switch return, unit tests
• This function will be extended to handle both checkout and vault returns in later PR along with distinction between
  appSwitchCheckoutCompletion and appSwitchVaultCompletion
  There will also be enums persisted after app switch launch of which flow was launched,
  or discerned implicitly by which completion was persisted.
• This function allows testing of app switch flows for upcoming PRs:
  • Toggling flows between web checkout and app switch checkout
  • Implementing the same flows for PayPal vault without purchase
  • Singleton class for routing incoming url's from app switch to appropriate class
  • Implementing controls to make sure no app switch attempts are made before initial launch completion

Checklist

• Added a changelog entry

Authors

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

• @KunJeongPark