Allow generating multiple keys in `sop generate-key[s]`?

(Split out from #112 (closed), and analogous to #99 (comment 2100646405)).

When setting up OpenPGP right now (or in the near future), you might want two keys: a v4 key (for compatibility) and a v6 key (that can soon use PQC, for added security).

And, you might want them to be bound together by @andrewgdotcom's replacement key subpacket, to show that they belong together.

Aand.. the user might not know that they "want" all that, it might just be that the implementation thinks that's a good idea.

So, it might be useful for sop generate-key (or a new command with the plural generate-keys?) to be able to generate this in one go, rather than having to run three commands (and having to know exactly what's the best practice at this current moment) to achieve that.