[bluez] No way to enable experimental features

[Koen Vervloesem]

I'd like to enable experimental features in the BlueZ snap to allow passive scanning, but the only setting I can configure is debug.

I'd like to be able to enable experimental features with something like:

```
snap set bluez experimental=true
```

[Tony Espy (awe)]

As I understand it, simply adding the experimental flag isn't going to get you what you want (i.e. passive scanning), as I'm pretty sure you'd also need for additional configuration for the additional features you'd like to enable, so I've updated the status to Incomplete and Importance to Wishlist.

If you cleanup the description/summary, we can mark it as Confirmed, but frankly unless there's any commercial interest in enabling experimental features, it's doubtful we'll implement this. That said, the source code is readily available and patches are always welcome!