Migrate PostgresBootstrapper to AwsCustomResource class

[leothomas]

• Ensure that both onCreate and onUpdate are implemented (ref in order to ensure that we can update the bootstrapper (and apply an updated bootstrapping logic to the database)
• Implement that actual bootstrapping itself through pypgstac's migrate functionality. (The bootstrapper currently executes the same SQL file as pypgstac's migrate function, however using the migrate function ensures that if pgstac is updated, the most recent migrations will be applied (as opposed to the version 0.3.4 , which is hardcoded in custom_resource/handler.py

[anayeaye]

PR #21 covers part of this ticket. The update adds pypgstac's run_migration function to migrate to pgstac 0.4.3. Verified pypgstac statement in log Migrating from 0.3.4 to 0.4.3..

We still need the onCreate and onUpdate implementations, the current delta-backend still uses conditional logic based on the event json passed to the handler.