--cosigner option?

Related to #12 & #13 & to LetheKit:

I think one of the most common uses for us (Blockchain Commons) is to get the 48h/0h/0h/2h cosigner derivation into the short text & a UR QR form (with the optional label and birthday), suitable for offering or receiving as part of a Policy Map -> Account Map process. Thus could maybe be a --cosigner shortcut option for keytool.

Currently I don't think we can use `crypto-account` for this, so we need to puzzle that through.

Note that this does not solve the problem of cosigner reuse: later we figure out a good way to have different cosigners for different Account Maps we'll need to puzzle that through as well. But a simple `--cosigner` option will help a lot.

Related, we may want to explicitly add this 48h/0h/0h/2h cosigner UR to LetheKit, as it long-term may be the most common export option from an offline seed.

/cc @gorazdko @wolfmcnally @Fonta1n3