We could expose more information about SK ssh keys

[directionless]

Digging into SK keys, it turns out they have options and we could expose them in our various key info tables.

The disk storage format is documented at https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.u2f and I think the flags themselves are https://github.com/openssh/openssh-portable/blob/82662d562cf54829df8a941cdfb2fd307e1d9a90/sk-api.h#L26

Note that we can only read this for unencrypted keys. Which might make the table output a little ugly.

If we did this, it would be around https://github.com/kolide/launcher/blob/9ead31a59083b3a805085e3e96957f9b06fff469/ee/keyidentifier/openssh.go#L48