* feat: Course Apps API

This adds a new concept called course apps. These are exposed via a new
"openedx.course_app" entrypoint, which helps the LMS and studio discover such
apps and list them in a new rest api for the same.

These course apps will drive the pages and resources view in the course authoring
MFE. This system will track which apps are enabled and which are disabled. It
also allows third-party apps to be listed here by using the plugin entrypoint.

* Apply feedback from review