[datadog_dashboard] Add slo datasource to dash widget formulas and functions query

[zhengshizhao]

To add terraform support to add slo datasources to Formulas and Functions query in widget.
jira ticket https://datadoghq.atlassian.net/browse/SLOS-1386

related [open-api PR] https://github.com/DataDog/datadog-api-spec/pull/2201

tested provider in local to create/update dash:

resource "datadog_dashboard" "slo_dashboard" {
  title        = "zhengshi test"
  description  = "Created using the Datadog provider in Terraform"
  layout_type  = "ordered"

  widget {
    timeseries_definition {
      request {
        formula {
          formula_expression = "a"
          alias = "slo"
        }
        query {
          slo_query {
            data_source = "slo"
            slo_id      = "51991ecc819552258314b808a80840c2"
            measure       = "slo_status"
            group_mode = "overall"
            name = "a"
          }
        }
      }
    }
    widget_layout {
      height = 5
      width  = 4
      x      = 4
      y      = 5
    }
  }