Add awareness of whether scanners have been enabled on Security Dashboard - Backend

backend issue for parent issue: #214392 (closed)

Implementation Outline

• Implement a graphQL API query with new type and resolver in ee/app/graphql. The query should look as follows

Sample Query:

{
  project(fullPath: "ssarka/demo-historic-secrets") {
    securityScanners {
      enabled
      available
      pipelineRun
    }
  }
}

Sample Result:

{
  "data": {
    "project": {
      "securityScanners": {
        "enabled": ["SAST","DAST"],
        "available": ["SAST", "DAST", "CONTAINER_SCANNING","SECRET_DETECTION", "DEPENDENCY_SCANNING"],
        "pipelineRun": ["SAST", "DAST"]
      }
    }
  }
}

• Write specs(RSpec) for the changes
• Update documentation in https://docs.gitlab.com/ee/api/graphql/reference/