scanner: detect OF devices in Nearby state

[malmeloo]

Breaking change to allow detection of 'nearby' devices that are not broadcasting full FindMy keys. These can be useful to detect whether a device is currently nearby.

Proposed API

OfflineFindingDevice is now either NearbyOfflineFindingDevice or SeparatedOfflineFindingDevice.

Both device types will have the following fields:

• mac_address
• status
• discovered_at
• additional_data

NearbyOfflineFindingDevices will have no additional unique fields.

SeparatedOfflineFindingDevices will have the following unique fields:

• hint
• all HasPublicKey fields (adv_key_bytes, hashed_adv_key_bytes, etc.)

Comparing devices against keys

Both subclasses will get an is_from method (name TBD) which will allow comparisons against keys and AirTag devices. The method will probably accept any HasPublicKey subclass (KeyPair, SeparatedOfflineFindingDevice) as well as any RollingKeyPairSource (FindMyAccessory). For this last one, the detection timestamp must be saved.

Resolves #46