Add an option to create a scoped signing-key in one command

[BZValoche]

What motivated this proposal?

Currently, to create a scoped signing-key, one hase to call nsc --sk generate, note the (non machine parseable) ouput, then call the command nsc --sk KEYJUSTCREATED --role blah.

What is the proposed change?

Allow the use of the --role parameter when using nsc --sk generate, so the key is immediately scoped, and can be referred by its role further in the key deployment process.

Who benefits from this change?

Using decentralized authentication for NATS is a very good feature. Scoped signing-keys is even better.
The current way the creation is implemented is error prone, because you either depend on one's copy/paste skills, or parsing an stdout output which you have no warranty that it won't change in the future.

What alternatives have you evaluated?

I need to check the generate return code, and if it is valid, grep / awk / sed the output to get the key argument to pass to the next nsc call. Very inefficient.