Countdown sensor "period" type

[wigster]

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.

For many devices, I typically set up something like a lifetime total runs/runtime sensor (always incrementing) and a twin sensor which counts down from some default. The typical use case is for example the number of cycles of the dishwasher with the twin sensor counting down from 30 to 0 which is when I alert myself to do some cleaning.

Describe the solution you'd like

Ideally a new type of "period" could be added to the list, which would take some default initial value and just count down in time together with the main sensors

Describe alternatives you've considered

I've been hardcoding a value template for a Measureit time sensor, e.g. {{360000-value}}, to get a countdown of 100 hours of use between servicing for my extractor fans.

Additional context

Ideally a reset to default button could be provided as standard in the more-info dialogue, a la the counter. entities.

[danieldotnl]

This is a duplicate of #155 but I'll keep it open for a while to see if others support your request.