Support cloudwatch metrics with multiple dimensions

Use-Case

Right now the cloudwatch scaler only lets you specify a single dimension name and value. This means you can't scale on any metrics with multiple dimensions.

Specification

It would be great to be able to specify a list of dimensions in the ScaleTrigger:

- type: aws-cloudwatch
  metadata:
    # Required: namespace
    namespace: MyCustomNamespace
    # Required: At least one dimension name
    dimensions:
    - Name: QueueName
      Value: keda
    - Name: env
      Value: dev
    metricName: AGE
    targetMetricValue: "2"
    minMetricValue: "0"
    ...