Add meta_key property to Coupons and Products

[n7studios]

Summary

Improves the CKWC_Admin_Post_Type class by adding a meta_key property, and dynamically loading the view based on the Post Type (e.g. Coupon/Product) being edited.

This allows for easier extension when adding fields to the Order Post Type in this PR.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)