Produce a list of all Slots in all MFEs in an org

[feanil]

As a part of the documentation process for named releases, it's useful to get a list of all slots that are defined in all MFEs in the org.

Slots are defined in a frontend repo in a very particular way so we can probably produce what we need by doing a search and outputting a list. We want a script that will find this data using the GitHub api and output in a format that is clean and easy to compare with output from a different time or a different GitRef.

Acceptance Criteria

• A simple script that lists the name of each folder under the src/plugin-slots folder in every repo in the openedx github organization.
• The user should be able to pass in a git tag or branch name and then it would only look only at repos that have that ref for plugin slots on that ref.
• The documentation for the Release Notes Process should be updated so that it has instructions to run this script on the current and previous release branch and document the new slots.