Adhere to WP code standards
Add order (desc/asc) to REST API customers endpoint

Part of #5720
Add orderby filtering to REST API resource endpoints

Adds orderby filtering to all endpoints using default WP orderby
parameters.

Closes #5720
Allow using meta_value_num with resource orderby

Part of #5720
Merge pull request #5914 from maxrice/feature-issue-5720

Add orderby filtering to REST API resource endpoints