feat: GATT service to disable bluetooth

[WhyNotHugo]

This is a feature request for a GATT service via which a client can disable Bluetooth on the Pinetime.

After the Pinetime is turned on, its Bluetooth is automatically enabled. I'm writing a tiny daemon for my host which will connect to the Pinetime and set the time for it. Once the time is set, I typically have no use for Bluetooth, and leaving it on drains my battery rather quickly. I'd like to be able to turn off Bluetooth from a remote host once the time is set.

The same functionality can be used by others with difference interfaces. For example, "companion apps" can have a button to turn off Bluetooth. Obviously, turning it back on requires interacting with the watch itself.