Refactor target_id and target_type from AuditEvent

Problem to solve

In an AuditEvent, the target_id and target_type is stored within a JSON hash. This is difficult and slow to query and has resulted in data inconsistency within the audit_events table.

Proposal

See parent proposal: #220116 (closed)

• Create target_id column

• Create target_type column

• Refactor write path to use new columns

• Refactor read path to handle historic data, where possible.

Links / references