Retrieve connected devices

[challiwill]

#259

[pyang2045]

@challiwill It appears this PR doesn’t have a reviewer yet

[challiwill]

@pyang2045 That's correct. I was hoping to get feedback from @qdot following the conversation in issue #259, but I figure we've all been busy with other commitments.

[szymonlesisz]

im not sure if retrieved connected peripherals should be treated as discovered.

i've played around this for a little bit and from my experience on macos if device is connected to another app you cant really use it, like for example disconnect it and im pretty sure i had some issues with writing or reading from it.
i guess this is the reason why corebluetooth is hiding it from the scan results.

with your changes it would act as it is discovered, connected and ready to go.

similar case is with retrieving peripherals by identifiers
if device is known (paired) but it is not advertising it will also not be visible in scan results.

instead i'm proposing less platform-specific approach and create Adapter.retrieve_peripherals() with options (by services, by identifiers, ...by other_future_params)

what dou you think? @qdot? @qwandor?