Background migration to insert data into new audit events tables

Background

As part of &13370 cells migration, we will need to split our audit_events table into 4 new tables:

1. user_audit_event
2. project_audit_event
3. group_audit_event
4. instance_audit_event

The objective of this issue is to start transferring current audit events data into new tables. We will be enabling sync of live data into new tables with the issue: #454175 (closed) So this background migration should only sync the data not present in these new tables.

Proposal

We can use batched background migration https://docs.gitlab.com/ee/development/database/batched_background_migrations.html for this.