feat: add user keys

[caarlos0]

This PR allows the user to add its own keys to their charm account.

This includes yubikeys or anything else the x/crypto libs support.

This is still not using the actual user keys for anything... for that we would probably want to support ssh agents et al as well

Likely depends on #121

[bashbunni]

yubikeys >:o

[caarlos0]

@toby would love your feedback in this one as well

[caarlos0]

changed it to a draft because a lot of things don't work, and some can't really work the way we're doing things today

so... back to the drawing board :D

[bashbunni]

oof