Add Dagrun stats and duration to Dagrun endpoint

[sollymay]

Hi Airflow team! First of all as I always try to do with any open source project I wanted to thank you for ALL the time you put into this project. We really appreciate your efforts.

I wanted to suggest an idea for the Dagrun API endpoint. Ideally, would like to include duration on the Dagrun details. Task instance does have duration but Dagrun does not, so would be awesome to have that for feature parity.

Additionally it would be great to have statistics on a particular DAG for all its Dagruns, like min duration, max duration and average duration as this would help with observability and reporting purposes. If this is not possible then duration would suffice as it can be calculated.