Issue #2145041 by tim.plunkett: Allow dynamic routes to be defined via a callback.

core/core.services.yml

...lib/Drupal/Core/EventSubscriber/AccessRouteSubscriber.php

...rupal/Core/EventSubscriber/EntityRouteAlterSubscriber.php

.../Drupal/Core/EventSubscriber/ParamConverterSubscriber.php

core/lib/Drupal/Core/Routing/RouteBuilder.php

core/lib/Drupal/Core/Routing/RouteSubscriberBase.php

core/lib/Drupal/Core/Routing/RoutingEvents.php

...lib/Drupal/config_translation/Routing/RouteSubscriber.php

...ules/content_translation/content_translation.services.yml

...translation/Routing/ContentTranslationRouteSubscriber.php

core/modules/field_ui/field_ui.services.yml

.../field_ui/lib/Drupal/field_ui/Routing/RouteSubscriber.php

core/modules/image/image.routing.yml

core/modules/image/image.services.yml

...dules/image/lib/Drupal/image/Routing/ImageStyleRoutes.php

core/modules/rest/lib/Drupal/rest/Routing/ResourceRoutes.php

core/modules/rest/rest.routing.yml

core/modules/rest/rest.services.yml

...s/search/lib/Drupal/search/Routing/SearchPluginRoutes.php

core/modules/search/search.routing.yml