Add transition_time to Tradfri

The underlying library for the Tradfri component has been updated to 2.0: https://github.com/ggravlingen/pytradfri/releases/tag/2.0

The updated release brings among other changes (API change :frowning: ) also support for transition times (https://github.com/ggravlingen/pytradfri/commit/6f5b1936ab08adc7389ab7cd91b091ae5d1c1e34). Would be great if that could be supported by homeassistant.