New Access Mode: Pay for certificate (OR Group Expiration)#4

Requesting a “Free access, pay for certificate” access mode (or a group expiration feature) so learners can take a course free but pay only if they want the certificate, with certificate access expiring/renewing periodically. (Migrated from featureOS: https://learndash.featureos.app/p/new-access-mode-pay-for-certificate-or-group-expiration)