Primary Key Field for SLCItem

[awhicks]

We need to create a primary key field and update it in the specification

This should probably be an open-ended string field where we recommend the author use something identifying like the URL of the exercise's home (a github URL or the exercise's URL)

For example, in OpenDSA, one such example might be: https://github.com/OpenDSA/OpenDSA/blob/master/Exercises/Graph/GraphIntroSumm.html