App Switch / WebView toggle in SDK

[KunJeongPark]

Reason for changes

• implement app launch vs ASWebAuthenticaitonSession flow toggle for both PayPal web checkout and PayPal web vault

Summary of changes

• update demo app app links to our updated sandbox merchant server url
• implement patchCCOWithAppSwitchEligibility call and toggling of app switch or ASWebAuthenticationSession flows for PayPal vault and checkout
• handleRequestURL in PayPalWebCheckoutClient extended to include PayPal vault app switch return

TODO on future PRs:

• 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
• Analytics for app switch
• Changelog entries

Checklist

• [ ] Added a changelog entry

Authors

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

• @KunJeongPark