[Routing] Allow force-generation of trailing parameters using eg "/exports/news.{!_format}"

When a route is defined as `path: '/exports/news.{!_format}'`, we should force `_format` be defined in "defaults" and the generator should generate URLs with that default when none is provided (should work with any parameter of course.)

Better described in https://github.com/symfony/symfony/pull/28633#issuecomment-431766878