feat: add enrollment timestamp to at-risk learners (FC-0051)

[SoryRawyer]

This change adds the timestamp of when a learner enrolled in a course to the fact_student_status model. This field is then used in dim_at_risk_learners to support displaying a distribution of when at-risk learners enrolled in the course.

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[bmtcril]

This is the most recent enrollment time, right? So when a user changes status (upgrades to certificate or unenrolls and re-enrolls) the clock resets? We should probably include that detail in the model docs for this and fact_enrollment_status.

[SoryRawyer]

That's a good point. Yes, it should be the most recent enrollment action timestamp.

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[bmtcril]

Released as v3.22.0, tutor plugin PR is here: https://github.com/openedx/tutor-contrib-aspects/pull/763