r/pgsql-server-base: Add post-upgrade capability

The `postgresql-upgrade` script will now run any executables located in
the `/etc/postgresql/post-upgrade.d` directory.  This will allow making
arbitrary changes to the system after a PostgreSQL major version
upgrade.  Notably, we will use this capability to change the WAL-G
configuration to upload WAL archives and backups to the correct
version-specific location.

roles/postgresql-server-base/tasks/main.yml

@@ -29,6 +29,16 @@
   - postgresql-upgrade
   - systemd
 
+- name: ensure postgresql post-upgrade directory exists
+  file:
+    path: /etc/postgresql/post-upgrade.d
+    owner: root
+    group: root
+    mode: u=rwx,go=rx
+    state: directory
+  tags:
+  - postgresql-upgrade
+
 - name: ensure postgresql-upgrade service starts when needed
   service:
     name: postgresql-upgrade