feat: support --severity-threshold for calculating exit code

[thisislawatts]

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What does this PR do?

Adds support --severity-threshold when determining the exit code. This means that you can use snyk code test --severity-threshold=high with the go based workflow and get the success exit code if only low severity issues are found.

How should this be manually tested?

Run the freshly built binary against a project that only contains low|medium severity issues, in this case I am running against https://github.com/snyk-fixtures/shallow-goof where .gitignore has been removed after installing so the scanner runs over node_modules.

~/snyk/cli/binary-releases/snyk-macos-arm64 code test ~/snyk/shallow-goof --org=<org-uuid-with-feature-flag> --severity-threshold=medium
echo $? // Should be 0

What are the relevant tickets?

CLI-304