Add namespace/project/user context to container_registry Snowplow events

Problem

Snowplow events for container_registry:notification do not collect namespace/project/user context information, making it impossible to link events to customer accounts or aggregate usage by company.

Affected Events

The following container registry events are missing context data:

• push_tag
• push_repository
• delete_tag
• delete_repository
• create_repository

All fall under the container_registry:notification event category.

Impact

Without this context data:

• Cannot attribute container registry usage to specific customers
• Cannot aggregate metrics by company/namespace for the Package North Star Metric dashboard
• Historical data cannot be backfilled once instrumentation is fixed

Solution

Add instrumentation to collect namespace, project, and user context for all container registry Snowplow events.

Related

• Issue: https://gitlab.com/gitlab-data/product-analytics/-/issues/3103 (New Package stage northstar metric)
• Discussion: https://gitlab.com/gitlab-data/product-analytics/-/issues/3103#note_2870573806