Add CLI tool to signature crate to check Balrog results

[oskirby]

Description

Following up from #11458 - the issue would have been easier to diagnose if there was a convenient tool that could check a Balrog update URL and print out some diagnostic details of the validation process. So, let's add such a tool as an example of the signature crate.

Reference

Related to: #11458
JIRA Issue: VPN-7720

Checklist

• My code follows the style guidelines for this project
• If bug/feature is at all notable, I've added a draft release note as a comment in addons/strings.yaml.
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed