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/files/postgresql-upgrade.sh

@@ -38,3 +38,13 @@ runuser -u postgres -- pg_upgrade \
     -o '-c restore_command= -c archive_command=' \
     -O '-c restore_command= -c archive_command=' \
     -k
+
+for f in /etc/postgresql/post-upgrade.d/*; do
+    if [ -x "${f}" ]; then
+        printf 'Running post-update script: %s\n' "${f}" >&2
+        "${f}" || {
+            printf 'Post-update script %s failed\n' "${f}" >&2
+            continue
+        }
+    fi
+done