BleakScanner: Add async iterator scanning capability

[bojanpotocnik]

Enable using BleakScanner() as an async iterator (async for), combining detection_callback and filtering functionality. BleakScanner().devices() method has the same functionality with option to provide timeout.

I noticed that almost every time I use bleak, one of the first things is to create scanner method which combines discover() and find_device_by_filter() functionality "in concurrent fashion", without the need of using callbacks and queue. This enables reporting scanned devices immediately while giving all the filtering flexibility, even when scanning for multiple devices with different filters.

[dlech]

Thanks for this. This is something that I have been wanting to add for a while.

I have made some suggestions on how we could simplify the implementation and usage a bit.

[dlech]

We also need to expand the "Getting discovered devices and advertisement data" section in scanner.rst so that the new method is included in the documentation.

[bojanpotocnik]

The last two comments are here so you can see the alternative implementations - I can add any of them back if desired.

[dlech]

Thanks for all of the work on this! It was more complicated than I expected.