Add Course Expiration Date

Add an option to set the date that a course expires to course expiry. Currently, you can only choose how many days/weeks/months a student has access. It is a highly requested and expected option to choose the exact date the course ends.

One example: https://senseilms.mystagingwebsite.com/course-access-expiration-is-here/#comment-222

This seems like a pretty quick project that can be added to Pro at any time.

Start by tracing the existing course-access expiration implementation and how duration-based access is stored and checked. Add support for an exact course expiry date alongside the existing days, weeks, and months options, then verify that student access ends on the selected date.

Written by the indexing model from the issue text.