postgres: Add pgBackRest integration

This serves to supersede rsnapshot, which has severe performance issues.

postgres/defaults/main.yml

@@ -3,6 +3,12 @@
 postgres_pgdg_repo: false
 postgres_rsnapshot: false
 postgres_commvault_compat: false
+postgres_pgbackrest: false
+postgres_wal_archive: "{{ postgres_commvault_compat or postgres_pgbackrest }}"
+
+pgbackrest_retention_diff: 7
+pgbackrest_retention_full: 2
+pgbackrest_frequently_minute: "*/15"
 
 ### Will be the version of the `postgresql` package if undefined
 # postgres_version: 15