Plugin: add aggregated completion events

[pomegranited]

Completion events are standard, openedx events, so supporting them should be part of the Aspects core, cf #88.

OpenCraft maintains the completion aggregator plugin app which currently lacks support for tracking events.

Contribution open-craft/openedx-completion-aggregator#173 adds tracking events for the aggregated completion states, and openedx/event-routing-backends#377 proposed pulling them into the ERB. But since these events aren't openedx events, we'd rather keep the event routing logic with the aggregator app.

• Get feat: add tracking logs open-craft/openedx-completion-aggregator#173 tested and merged: Add tracking logs open-craft/openedx-completion-aggregator#206
• Refactor feat: add aggregator events event-routing-backends#377 into a https://github.com/open-craft/openedx-completion-aggregator PR: feat: adds xApiTransforms for completion aggregator events open-craft/openedx-completion-aggregator#205
  May require some refactoring of feat: add aggregator events event-routing-backends#377 to support sharing event routing and testing logic.
• Create a dbt extension that adds aggregated completion views to aspects-dbt. feat: transforms aggregated completion "progress" events into a new fact open-craft/dbt-completion-aggregator#1
• Create a tutor plugin that adds custom tab / charts to display aggregated completion information in instructor dashboard Initial plugin setup open-craft/tutor-contrib-completion-aggregator#1
• Document this example of how to create an Aspects plugin in the How Tos.
  Ensure caveats of the aggregator are noted (cf sync vs async) if providing installation instructions.
• Optional: create a management command to read the completions aggregates from the database and send them in a way that we can backfill this data. cf Feat: Add completion events #88 for ideas on how to approach this.

[Ian2012]

Create a tutor plugin that adds custom tab / charts to display aggregated completion information in instructor dashboard

@pomegranited For this task there should be some work on Aspects that allow you to re-use the import_assets_helper module on your tutor-plugin so that it can facilitate it the serialization and organization of plugins. In theory it should work out-of-the-box but let me know how that goes.

Another concern is how will you append a tab for the completions to the Instructor Dashboard on a patch.