Adds an Additional way of identifying order id on amazon return

[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:

Provides an additional method of identifying the order id , for which we need to complete the checkout session. Introduces 2 new filters. apa_merchant_metadata_reference_id - For enabling merchants to alter the merchantReferenceId provided to Amazon apa_merchant_metadata_reference_id_reverse - For those merchants that use the above, or alter the plugins default value for merchantReferenceId in any way, a way to provide the order id back to the plugin.

Closes woo#134 .

How to test the changes in this Pull Request:

This issue was not reproducible, logs we saw from various merchant sites suggested that on the method handle_return sometimes the WC()->session->order_awaiting_payment variable was not set.

Other information:

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

Changelog entry

Add - Additional way of identifying order id after amazon update_checkout_session Add - 2 new filters. Control over the value of merchantReferenceId provided to Amazon, but also a way to retrieve the original.

[dpanta94]

opened pr in woo repo pull#167

[dpanta94]

Accepted in woo repo!