Add `identifer` to response to pass as metric

This MR introduces another field identifier to https://docs.gitlab.com/ee/api/graphql/reference/#securitytrainingurl so that we can use it in Pass metric identifier for training link clicked on vulnerability details page. We named it identifier not identifierName/identifierId` so that we can hold any field from Vulnerability identifier to pass in metric.

Related to #357569 (closed)