Test: V6 Instance Provider

[EvanReinstein]

This PR/branch contains V6 @paypal/react-paypal-js changes. In order to run this example correctly you'll need to create a symlink between the branch with these changes and this branch with changes for react-paypal-js.

Step by Step Instructions:

1. Make sure you have cloned down the sample integrations repo and the paypal-js repo.
2. Pull changes from the v6-web-sdk-sample-integration branch: test/DTPPCPSDK-3479-provider.
3. Run npm install to install the latest @paypal/react-paypal-js version.
4. In your paypal-js directory pull changes from feature/DTPPCPSDK-3479-provider.
5. Run npm install and npm run build and navigate to the react-paypal-js package root. Run npm run prepack and then run npm link.
6. In the same terminal navigate to the v6-web-sdk-sample-integration directory and to the React example.
7. Run npm list @paypal/react-paypal-js in order to confirm that the package is pointing to node modules
8. Run npm link @paypal/react-paypal-js and then run npm list @paypal/react-paypal-js to confirm that the package is pointing to your local version.
9. Run npm run start and confirm that the SDK, PayPal and Venmo buttons are loading and working correctly.