feat: include username in event data

[BryanttV]

Description

This PR adds the username field to the data of the events.

Motivation

Currently, the events emitted by openedx-completion-aggregator only include the user_id, which creates a significant limitation for integration scenarios with external services.

According to the Open edX Events documentation regarding integration with external systems, TRACKING_EVENT_EMITTED events are sent through the event bus to be consumed by third-party services. The issue is that with only the user_id available in the event, external consumers face an incomplete workflow.

This change follows the existing pattern used in Open edX, where events include the username:

• NOTIFICACION_PREFERENCE_UNSUBSCRIBE
• EMAIL_DIGEST_SENT
• BULK_EMAIL

Testing Instructions

Using Tutor:

1. Install the plugin using this branch.
2. Configure the Event Bus (Redis or Kafka).
3. Check the emitted event that includes the username field.

Merge Checklist

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed
• PR author is listed in AUTHORS