postgresqlTestHook: add postgresqlExtraSettings variable

Add `postgresqlExtraSettings` to allow to setup the `postgresql.conf`.
This would be useful in cases where we need to set
`shared_preload_libraries`.

pkgs/build-support/setup-hooks/postgresql-test-hook/postgresql-test-hook.sh

@@ -56,6 +56,8 @@ EOF
   echo 'initializing postgresql'
   initdb -U postgres
 
+  echo "$postgresqlExtraSettings" >>"$PGDATA/postgresql.conf"
+
   # Move the socket
   echo "unix_socket_directories = '$NIX_BUILD_TOP/run/postgresql'" >>"$PGDATA/postgresql.conf"