Feature/add estimated order amount pay button

[dpanta94]

All Submissions:

• [x] Does your code follow the Extendables standards?
• [ ] Have you written new tests for your changes, as applicable?
• [x] Have you successfully run tests with your changes locally?

Changes proposed in this Pull Request:

Add estimatedOrderAmoun attribute to Amazon Pay button.

How to test the changes in this Pull Request:

1. Add a product to the cart.
2. Make sure that amazon pay button is active.
3. Try to update the cart items (change the quantity of the added item, add a new item or remove the existing one)
4. The following message is logged on browser's console (sandbox mode): Estimated order amount for sandbox is {"amount":"XXX","currencyCode":"XXX"}

Other information:

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changelog entry

Add - Estimated order amount attribute on pay button.