Add "email" or unique user "SSO ID" to the audit events API responses

Proposal

We are looking at adding the audit event author email and/or unique SSO ID to the instance audit events export. To keep consistency within the audit events feature, we should add the same information to the API responses for retrieving audit events at the instance, group, and project levels.

• Instance audit event API https://docs.gitlab.com/ee/api/audit_events.html#instance-audit-events
• Group audit event API https://docs.gitlab.com/ee/api/audit_events.html#group-audit-events
• Project audit event API https://docs.gitlab.com/ee/api/audit_events.html#project-audit-events