Expose a additional PG config options via juju config

Hi team, Mohammed and I discussed a few more LP PG config options, and we aligned on exposing the following PG config options via juju config.

1. `maintenance_work_mem`
2. `hot_standby_feedback` - Mohamed mentioned this config option is already planned to be exposed.
3. `work_mem`
4. `random_page_cost` - There was some discussion about `random_page_cost` being automatically tuned based on the underlying disks, but since that functionality is not currently implemented, it should be exposed as a configurable parameter. Please double-check with Mohamed on this.

Let me know if you need anything :)

Thanks!