Allow spec'ing backends from pool config or call site about flame HOT 3 CLOSED

 children = [
  {DNSCluster, query: Application.get_env(:grid_importer, :dns_cluster_query) || :ignore},
  {
    FLAME.Pool,
    name: SomePoolName,
    min: 0,
    max: 1,
    max_concurrency: 1,
    single_use: true,
    log: :debug,
    idle_shutdown_after: 5_000,
    backend: {
      FLAME.FlyBackend,
      token: System.fetch_env!("FLY_API_TOKEN"),
      cpu_kind: "shared",
      cpus: 8,
      memory_mb: 16384,
      env: %{
        "DNS_CLUSTER_QUERY" => System.fetch_env!("DNS_CLUSTER_QUERY"),
        "MIX_ENV" => System.fetch_env!("MIX_ENV")
      }
    }
  }
]