Hybrid transport: Switch to event-driven discovery

[AlfioEmanueleFresta]

Changes

• Switches from a periodic polling approach, to an event-driven discovery. It uses btleplug's event system to wait for a BLE advertisement. This is guaranteed to occur after the QR code is generated, making this safe.
• Fixes iOS discovery limitation, by removing UUID filter for BlueZ device discovery. This seems to be a limitation in BlueZ (it wouldn't be the first), that I could not find a workaround for. It should have no significant downside, however, as discovered devices that don't include the relevant service data are ignored (we do not attempt connecting to them).

Testing

Tested on an iPhone, and Android 15. Example webauthn_cable succeeds consistently.

[msirringhaus]

I've left some minor comments

[AlfioEmanueleFresta]

Thanks @msirringhaus! Addressed your comments.

[msirringhaus]

Thank you!