[DatadogDashboard] Support for yaml syntax for widgets

[starlightromero]

Currently DatadogDashboard.spec.widgets accepts a JSON string. However, after only a few widgets this becomes cumbersome and hard to maintain, both from a readability standpoint and a validation standpoint. JSON strings cannot natively be linted or validated with standard Kubernetes tooling. Instead, widgets should accept yaml which then in the controller, is converted to json.

Before:

  widgets: |
    [
      {
        "definition": {
          "title": "Pod Count",
          "type": "query_value",
          "requests": [
            {
              "response_format": "scalar",
              "queries": [
                {
                  "data_source": "metrics",
                  "name": "query1",
                  "query": "sum:kubernetes.pods.running{cluster_name:example-cluster,service:$app.value,env:$env.value}",
                  "aggregator": "last"
                }
              ]
            }
          ],
          "precision": 0
        },
        "layout": {"x": 0, "y": 0, "width": 2, "height": 2}
      }
  ]

After:

widgets:
  - definition:
      title: Pod Count
      type: query_value
      requests:
        - response_format: scalar
          queries:
            - data_source: metrics
              name: query1
              query: sum:kubernetes.pods.running{cluster_name:example-cluster,service:$app.value,env:$env.value}
              aggregator: last
      precision: 0
    layout:
      x: 0
      y: 0
      width: 2
      height: 2