Add a sensor with the connected bluetooth device battery level

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

Sometimes I leave the house with my bluetooth headphones low on battery and then the battery runs out while I'm out in the middle of nowhere. I can't listen to music. It's a disaster.

**Describe the solution you'd like**

I'd like a sensor that shows the battery level of the connected bluetooth device. Then I'd create an automation that sends me a notification that the battery is low when I get home so that I can plug it in.

**Describe alternatives you've considered, if any**

Haven't considered any.

**Additional context**

None.