Add service definition for SyslogFormatter

[sarim]

From https://github.com/symfony/recipes/issues/1111 & https://github.com/Seldaek/monolog/pull/1689 I found out about the (newish) syslog formatter that can be viewed nicely with lnav. While trying to use it, I see there is no service created for this in monolog-bundle. Users can obviously make their own service in their app's services.yaml, but seeing other formatters exposed and discussion about adding it via recipe, I thought it would be good idea to create the service from monolog-bundle.