Add SecretDetectionScanEvent for observability

What does this MR do?

Adds SecretDetectionScanEvent type with event name collect_secret_detection_scan_metrics_from_pipeline for Secret Detection observability.

Why is this needed?

Secret Detection analyzer needs to collect scan performance metrics. This follows the same pattern as collect_sast_scan_metrics_from_pipeline for consistency.

Event Schema

{
  "event": "collect_secret_detection_scan_metrics_from_pipeline",
  "property": "scan-uuid",
  "label": "FetchRange",
  "value": 0,
  "version": "7.20.0",
  "exit_code": 0,
  "time_s": 45,
  "commit_count": 127,
  "repo_size": 52428800
}

Related

https://gitlab.com/gitlab-org/gitlab/-/issues/536366+