Transitions for (speed) fan component

**Describe the problem you have/What new integration you would like**
I would like to specify a transition for the fan speed in seconds.

**Please describe your use case for this integration and alternatives you've tried:**
While it is possible to achive this using template instances, globals and an interval-loop a simpler solution would be very helpful.

-> I control the fans to my server room using ESPHome and during hot days the higher speeds are very binary. On. Off. On. Off... Some kind of transistion would make it much more convenient.

**Additional context**
Similar to https://www.esphome.io/components/light/#config-light ?

- default_transition_length
- flash_transition_length