[datadog_monitor] Add cost alert monitors

[klara-kr]

Adding terraform support for the cost alert monitors

I did have an example in https://github.com/DataDog/terraform-provider-datadog/blob/6fe11d82a1ca55992e511236236577c9f31af879/docs/guides/monitors.md but it was removed when I ran make docs -- let me know if I should add it back

## Cost Monitors

```terraform
resource "datadog_monitor" "process_alert_example" {
  name    = "Cost Alert Monitor"
  type    = "cost alert"
  message = "Cost Change increased more than 50$ in the last 30 days"
  query   = "formula('query').last('30d').change('absolute') > 50"
  monitor_thresholds {
    critical = 50
  }
  
  variables = [
    {
      name  = "query"
      value = "sum:aws.cost.amortized{*} by {service}.rollup(sum, daily)"
      aggregator = "sum"
      data_source = "cloud_cost"
    }
  ]
  renotify_interval = 60
}