pw_software_update: Sign a bundle via CLI
pw_software_update: Sign a bundle via CLI

Add functionality for signing of bundle

Change-Id: Ic878b3d0df26c4a5514f382260d1aa6ab5e8ec7a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/120671
Reviewed-by: Eashan Singh <cad5129ea6204a187a17d71caa9c1eeccd0c65c4@google.com>
Reviewed-by: Ali Zhang <101974175ff73f104389b21e58229a7713cff21f@google.com>
Commit-Queue: Eashan Singh <cad5129ea6204a187a17d71caa9c1eeccd0c65c4@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed