Save audit events for start/stop user impersonation to group level

What does this MR do and why?

• Adds support for group namespace owners to see if an instance administrator has impersonated one of their users.
• When a user is impersonated by an instance admin, an audit event is now added to all group namespaces that the user belongs to.
• Another event is also added when the impersonation is explicitly ended.

Screenshots or screen recordings

Admin Audit Log	Group Audit Log

How to set up and validate locally

• Create a group and add a user to it.
• Impersonate that user, as an instance adminstrator.
• Stop the impersonation.
• Check the group audit log for a notification that the user has been impersonated, and that the impersonation has finished.
• Check the admin audit log for similar notifications and for copies of the group-level audit events too.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Relates to #300961 (closed)