Add address_type argument for backends

[javl]

Thanks for this wrapper. I wanted to use it with a completely different device, one that needs the 'random' address type instead of 'public'.

I've added an aditional argument to the bluetooth interface to allow this. The default is still 'public' so BluetoothInterface(backend, adapter) is equal to BluetoothInterface(backend, adapter, address_type='public') meaning scripts implementing btewrap in the 'old' way should still work the same, but it also allows you run run BluetoothInterface(backend, adapter, address_type='random') to use the random address type.

I've implemented this in all three backends but was only able to test it for bluepy and gatttool as pygatt doesn't seem to work on my system. I think it should work, maybe you can take a look and confirm this?

[javl]

Sorry for the sloppy commits, I'll close this pull request and create a new one with a simple commit

[coveralls]

Pull Request Test Coverage Report for Build 29

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