[FA] Set display_priority in postgres spec.yaml (DataDog#23273)

* Set display_priority in postgres spec.yaml based on field usage

* Add changelog entry for postgres

* Update display_priority in postgres spec.yaml (enforce host/port and username/password adjacency)

* Reorder fields in postgres spec.yaml by display_priority

* Regenerate conf.yaml.example for postgres

* Set display_priority on all fields including previously missing ones

* Sync conf.yaml.example with spec.yaml

Author: coignetp