Add migration to add create_at column in external_service_repo table

Created by: varsanojidan

Closes #27795 (closed)

Adding an additional row to the external_service_repos table to log when repos are added by users so we can keep track of the added times for the repos if multiple adds are made.

Example:

Test plan

Tested by running migration up and down against development environment, as well as adding a new repo and verifying the default created_at time was set correctly in the DB table