[datadog_dashboard] Add run-workflow widget definition

[DavidRobertAnsart]

Changes

• Add run workflow widget definition to dashboard resource.
• Add test and recorded cassettes for the new widget.
• Add doc.

How to test:

1. Build the provider locally.
2. Try this dashboard resource:

resource "datadog_dashboard" "ordered_dashboard" {
  title       = "Run workflow terraform dashboard"
  layout_type = "ordered"
  template_variable {
    name             = "Env"
    prefix           = "Env"
    defaults         = ["*"]
    available_values = ["prod", "staging", "test"]
  }
  widget {
    run_workflow_definition {
      title       = "My workflow widget"
      title_size  = "13"
      title_align = "left"
      workflow_id = "2e055f16-8b6a-4cdd-b452-17a34c44b160"
      input {
        name  = "env"
        value = "$Env.value"
      }
    }
    widget_layout {
      x      = 0
      y      = 0
      width  = 4
      height = 4
    }
  }
  notify_list = []
  reflow_type = "fixed"
}

[cswatt]

small nitpicky things

[DrkSephy]

Thanks for this contribution 🙌