Populate lineage_events created_at timestamp for every new event

[prachim-collab]

As part of issue: #2300 a new created_at column will be added to lineage_events table. Once that DB migration change is rolled out. We want to populate the new column with database server's UTC now() timestamp whenever the event is inserted in the DB. To achieve this /lineage events POST api should be updated.

[denimalpaca]

@wslulciuc what's the work here that needs to be done on the Marquez side?