[BE] Add GraphQL field according to schema design

Add GraphQL field according to proposed schema design.

query getGroupVulnerabilitiesByIdentifier(
  $fullPath: ID!
  $projectId: [ID!]
  $reportType: [VulnerabilityReportType!]
  $severity: [VulnerabilitySeverity!]
  $securityAttributesFilters: [AttributeFilterInput!]
) {
  group(fullPath: $fullPath) {
    id
    securityMetrics(projectId: $projectId, reportType: $reportType, securityAttributesFilters: $securityAttributesFilters) {
      vulnerabilitiesByIdentifier(severity: $severity) {
        identifierName
        bySeverity {
          severity
          count
        }
      }
    }
  }
}

This will only add fields without returning proper results, could be blank array so that we can work frontend part smoothly.