RFE: ability to use existing keys

[jku]

I created https://github.com/jku/playground-tuf-minimal as a minimal TUF implementation with just two separate keys.

This was not possible without manual editing as it's not possible to specify existing keys in
tufrepo edit root add-key
Adding that possibility for keys that already have a private key in privkeys.json should be pretty easy?