Add timestamp when max min occurred as atttributes

[droidgren]

Checklist

• I have filled out the template to the best of my ability.
• This only contains 1 feature request (if you have multiple feature requests, open one feature request for each feature request).
• This issue is not a duplicate feature request of previous feature requests.

Is your feature request related to a problem? Please describe.

I would like to get the timestamp when the max min occurred of a given sensor.
So you wouldn't need to create an additional sensor for this.

Describe the solution you'd like

These attributes would be added:

min_datetime:
Date and time of minimum value of processed values of source sensors.

max_datetime:
Date and time of maximum value of processed values of source sensors.

Describe alternatives you've considered

Additional context