Plugin: add aggregated completion events

Completion events are standard, openedx events, so supporting them should be part of the Aspects core, cf https://github.com/openedx/openedx-aspects/issues/88.

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

Contribution https://github.com/open-craft/openedx-completion-aggregator/pull/173 adds tracking events for the aggregated completion states, and https://github.com/openedx/event-routing-backends/pull/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 https://github.com/open-craft/openedx-completion-aggregator/pull/173 tested and merged: https://github.com/open-craft/openedx-completion-aggregator/pull/206
• Refactor https://github.com/openedx/event-routing-backends/pull/377 into a https://github.com/open-craft/openedx-completion-aggregator PR: https://github.com/open-craft/openedx-completion-aggregator/pull/205
  May require some refactoring of https://github.com/openedx/event-routing-backends/pull/377 to support sharing event routing and testing logic.
• Create a dbt extension that adds aggregated completion views to aspects-dbt. https://github.com/open-craft/dbt-completion-aggregator/pull/1#
• Create a tutor plugin that adds custom tab / charts to display aggregated completion information in instructor dashboard https://github.com/open-craft/tutor-contrib-completion-aggregator/pull/1
• Document this example of how to create Aspects plugins in the How Tos:
  https://docs.openedx.org/projects/openedx-aspects/en/latest/technical_documentation/how-tos/xapi_transforms.html
  https://docs.openedx.org/projects/openedx-aspects/en/latest/technical_documentation/how-tos/dbt_extensions.html
  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 https://github.com/openedx/openedx-aspects/issues/88 for ideas on how to approach this.