reports: easier accessory key fetching

[malmeloo]

This PR introduces some quality of life changes to make it easier to work with FindMy accessories:

• Key generation between time ranges is now directly supported using FindMyAccessory.keys_between;
• Accessory objects can now be used directly to retrieve reports;
• Accessories can be serialized from compatible .plist files;
• A few other minor bugfixes and QOL changes.

Still needs more testing.

This is one of the last changes before I consider AirTag support to be final, at which point #4 can probably be closed.