Add support for various CloudWatch Dashboard features

[immmapig]

The Dashboard construct is missing support to control liveData for the Graph widget. Within the graph widget, it is also not possible to configure showUnits and label for the y-axis.

The following is an example of a graph widget produced by the AWS console that configures all 3 of the configurations mentioned above:

{
    "view": "timeSeries",
    "title": "Engagement Events/5 Mins",
    "region": "us-east-1",
    "metrics": [
        [ "kennetdo-PrESSAppLambda", "OrderEventEngagementEvent", { "yAxis": "left", "period": 300, "stat": "Sum" } ],
        [ ".", "DigitalOrderEventEngagementEvent", { "yAxis": "left", "period": 300, "stat": "Sum" } ],
        [ ".", "AsinPageEngagementEvent", { "yAxis": "left", "period": 300, "stat": "Sum" } ],
        [ ".", "AVStopPlaybackEngagementEvent", { "yAxis": "left", "period": 300, "stat": "Sum" } ],
        [ ".", "TrackDownloadedEngagementEvent", { "yAxis": "left", "period": 300, "stat": "Sum" } ],
        [ ".", "TrackStreamedEngagementEvent", { "yAxis": "left", "period": 300, "stat": "Sum" } ]
    ],
    "yAxis": {
        "left": {
            "min": 0,
            "showUnits": false,
            "label": "Custom Label"
        },
        "right": {
            "min": 0
        }
    },
    "period": 300,
    "liveData": false
}

[immmapig]

Metric math support is also missing.