Create a GraphQL query to get severity override in vulnerability report page

What does this MR do and why?

Create a GraphQL fragment to get the severity override in the vulnerability report page only.

References

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Before	After

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Enable the feature flag (echo "Feature.enable(:vulnerability_severity_override)" | rails c) and (echo "Feature.enable(:enhanced_vulnerability_bulk_actions)" | rails c)
3. Navigate to the vulnerability details page (http://gdk.test:3000/[path/to/project]/-/security/vulnerabilities/[id])
   1. navigate to a project that has detected vulnerabilities
      • need a test project? clone this project and run a pipeline to generate some vulnerabilities
   2. in the sidebar, choose Secure => Vulnerability report
4. The mutation is still is another open MR, so you can mutate in you local GraphQL:

  vulnerabilitiesSeverityOverride(
    input: {
      severity: CRITICAL
      vulnerabilityIds: [
        "gid://gitlab/Vulnerability/XXX"
      ]
      comment: "This is a comment"
    }
  ) {
    errors
    vulnerabilities {
      id
      title
      severity
      webUrl
    }
  }
}

5. Verify you see an icon and tooltip on Vulnerability report page

Numbered steps to set up and validate the change are strongly suggested.

Related to #517832 (closed)