Add Custom Field API functions

[n7studios]

Summary

Adds the following API functions to provide API coverage for Custom Fields

• get_custom_fields()
• add_custom_field()
• add_custom_fields()
• update_custom_field()
• delete_custom_field()

Testing

• Tests for above functions included.

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)