SSC: Implement 'ssc appversion-vulnerability count' command

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Options:

• --from taking the appversion id
• --filterset taking a filter set by name or id (implement a FilterSetHelper class that allows for resolving filter set names and id's)
• Existing --field option can be used to get the issue counts for specific folder(s)

Together with issue templates, this allows for implementing functionality similar to quality/security gates:

• On SSC, you'd have issue templates that define security gate criteria in a dedicated filter set that has a single folder displaying for example all critical OWASP Top 10 issues that haven't been marked as 'Not an Issue'
• On the client side, you could do something like the following to test whether the number of vulnerabilities in the Quality Gate filter set doesn't exceed maxVulnerabilities:
  • [[ $(fcli ssc appversion-vulnerability count --from myApp:myVersion --filterset "Quality Gate" --fmt table --no-headers) > ${maxVulnerabilities} ]]

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Basic functionality has been implemented; we'll open a new issue if any enhancements are needed