Add a sub command to generate a node key file (#4884)

* Add a sub command to generate a node key file in the format required by a substrate node

* Update lock file

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Updates as per code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>