Add support to cancel Eco+ event

[rplankenhorn]

The API provides the ability to cancel Eco+ events:

https://developer.ecobee.com/home/developer/api/documentation/v1/operations/post-cancel-demand-response.shtml

This would be useful in Home Assistant because some users are not able to opt out of Eco+ and this could allow for an automation to cancel the event if it gets triggered.