Adds GraphQL queries for vulnerabilities

What does this MR do and why?

Adds GraphQL queries for vulnerabilities

Adds the vulnerability_namespace_statistic field, which exposes the number of vulnerabilities, by severity, for each group, returning zeroes and a null updated_at if there is no record in the vulnerability_namespace_statistics table. Also adds the vulnerability_statistic filed, which exposes the number of vulnerabilities, by severity, for each project.

Changelog: added EE: true

References

How to set up and validate locally

query SubgroupsAndProjects {
  group(fullPath: "gitlab-org") {
    descendantGroups(includeParentDescendants: false, first: 20) {
      nodes {
        name
        vulnerabilityNamespaceStatistic {
          updatedAt
          total
          info
        }
      }
    }
    projects(first: 10) {
      nodes {
        name
        vulnerabilityStatistic {
          updatedAt
          info
        }
      }
    }
  }

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