contrib: add a `kbs2 airdrop` script

[woodruffw]

kbs2 qr works nicely, but involves scanning an image (and thus having a picture of your credentials on your screen).

It'd be nice if we had another contrib script that allows users to Airdrop their credentials to a supported device (i.e., Apple devices).

Rough CLI sketch:

kbs2 airdrop gmail
kbs2 airdrop --id $some_device_id gmail

...where --id can be pulled from a default value in configuration.

In terms of actually using Airdrop, it looks like our best option is probably OpenDrop.

[woodruffw]

One option here: https://github.com/vldmrkl/airdrop-cli