nk3 secrets: Ramdomly generate challenge-response secret

[robin-nitrokey]

Users have to do something like this to set up a secret for use with KeypassXC:

$ nitropy nk3 secrets add-challenge-response 2 $(echo your-20-byte-secret | base32)

It would be more convenient to have an option that takes random bytes from the Nitrokey 3, performs the base32 encoding and then writes the slot.