Internal event schema for dependency firewall service metrics

What does this MR do and why?

Add dependency firewall internal events (package & container registry)

• Added EE event YAMLs:
  - ee/config/events/collect_dependency_firewall_metrics_on_package_upload_to_package_registry.yml
  - ee/config/events/collect_dependency_firewall_metrics_on_image_push_to_container_registry.yml
  - ee/config/events/collect_dependency_firewall_metrics_on_image_pull_from_container_registry.yml
  - ee/config/events/collect_dependency_firewall_metrics_on_package_download_from_package_registry.yml - ee/config/events/collect_dependency_firewall_metrics_on_forwarded_package_download_from_package_registry.yml

What each event records
- Identifiers: project/namespace and user (container pull uses namespace + user).
- Common additional properties:
- label: outcome enum — one of allowed, blocked-license, blocked-vulnerability, blocked-malicious, warn-license, warn-vulnerability, warn-malicious.
- property: auth type or cache status as appropriate (e.g., guest/user/deploy_token for package uploads; 1/0 cache hit/miss for container events).
- value: runtime in milliseconds.
- purl (package file) or image (container events) to identify the artifact. - Milestone: 18.9.
- Tiers: premium / ultimate (adjust if any event should be EE-only).

References

• https://gitlab.com/groups/gitlab-org/-/work_items/20584+s
• https://gitlab.com/gitlab-org/gitlab/-/work_items/587704+s
• https://gitlab.com/gitlab-org/gitlab/-/work_items/587703+s
• https://gitlab.com/gitlab-org/gitlab/-/work_items/587697+s
• https://gitlab.com/gitlab-org/gitlab/-/work_items/587696+s
• https://gitlab.com/gitlab-org/gitlab/-/work_items/587691+s

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