New sensor: low_battery_level_threshold

[danielbrunt57]

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.

It would be nice to have individual "low-battery" threshold values stored for each device to trigger low-battery notifications . I've not found a low-battery battery notification package or blueprint which addresses how different devices (like my front door lock) do not work well below 75%, others might work okay down to 40% while some may go as low as 20% or lower and still function.

Describe the solution you'd like

Create an additional sensor to store a low-battery level threshold value (%) for the device along with a service to be able to set/change the value, like the one for battery replaced date.

Describe alternatives you've considered

Digging inside existing solutions to hard-code exceptions for the low-battery alert threshold.

Additional context

Perhaps also consider adding an additional true|false sensor which becomes true when battery_level < low_battery_threshold.
Once a low-battery level threshold sensor exists, users could create templated helpers but having the integration do that itself would be useful!

[andrew-codechimp]

I like this idea and something else I'm working on at the moment will tie in well to this.
I'll just have to consider how a default threshold will work.

Moving it to discussion/ideas where I keep longer term features.