Adding Support for alternate product feeds into facebook, but still have order sync

[richardteachout]

Fixes #

[ X ] Do the changed files pass phpcs checks? Please remove phpcs:ignore comments in changed files and fix any issues, or delete if not practical. Changes proposed in this Pull Request: adding support for a site doing it's own product sync to facebook (outside of this plugin) to still get orders to sync in. I believe all my edits shouldn't affect a site which is already using the sync native to this plugin.

How to test the changes in this Pull Request: Create a feed outside of this plugin, post to facebook, create an order, and it should sync. All that needs to happen is to have the retailer_id == post_id (or == wc_post_id_)

Changelog entry Added some extra catches and functionality to allow a site using an external product sync, to still use this plugin for order synchronization.

[richardteachout]

If you're interested in committing this, I can also write up a readme /how to of my complete solution using this plugin natively, and what other functionality that I implemented to make it a complete sync solution.

[rawdreeg]

We no longer support FB checkout order sync. So, I will close this PR. Thanks.