Tweak/plugin header updates

[alexiglesias31]

All Submissions:

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

Changes proposed in this Pull Request:

Adds 3 new headers to every Request to the Amazon Pay API.

How to test the changes in this Pull Request:

1. Amazon Pay API requests should include the following extra headers.
   x-amz-pay-platform-version set to active WooCommerce's version
   x-amz-pay-integrator-version set to active Amazon Pay for WooCommerce plugin's version
   x-amz-pay-integrator-id set to the SP's (Solution Provider) ID.

Other information:

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

Changelog entry

Add - New Amazon Pay Platform related Headers

[kalessil]

LGTM