make x-defang-postgres have settings

[nullfunc]

Description

Add options for making snapshots/backups in managed RDS and having a maintenance window. Having a maintenance window will enable blue-green upgrading in AWS

Update x-defang-postgres in compose to be structured to have settings

• for maintenance window
• back up retention period
• snapshot restoration
• final snapshotting
• auto-save period

Linked Issues

Enable RDS Postgres version updating for PROD env

Checklist

• [X] I have performed a self-review of my code
• [X] I have added appropriate tests
• [ ] I have updated the Defang CLI docs and/or README to reflect my changes, if necessary