Add created_at indexed column to Lineage events table

[prachim-collab]

For analytics use case we need to incrementally copy and export lineage_events to DWH. For this use case there is no good way to identify incrementally created events in database. The current event_time in lineage_events table is client generated and can be back dated.
Proposing to alter lineage_events table to add a new created_at timestamp column. Also index this new column.
More discussion to be had regarding backfill and default values of this column.