feat: Add support for custom metric queries in customized_metric_spec

[mattaltberg]

Description

I added support for customized_metric_specification when using the metrics block, instead of using the current setup: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/appautoscaling_policy.html#create-target-tracking-scaling-policy-using-metric-math

Motivation and Context

This change adds support for metric math in queries, which is useful for things like SQS-queue size based autoscaling

Breaking Changes

No, since you can still specify a metric the old way

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

There was no example to test against, but the example code would look like so:

  autoscaling_policies = {
    "scale_consumer" = {
        policy_type = "TargetTrackingScaling"
        target_tracking_scaling_policy_configuration = {
          customized_metric_specification = {
            metrics = [{
                id = "m1"
                label = "Get the queue size"
                return_data = false
                metric_stat = {
                  metric = {
                    metric_name = "ApproximateNumberOfMessagesVisible"
                    namespace = "AWS/SQS"
                    dimensions = [{
                      name = "QueueName"
                      value = aws_sqs_queue.queue.name
                    }]
                  }
                  stat = "Sum"
                }
            },
            {
              id = "m2"
              label = "Get the number of running tasks"
              return_data = false
              metric_stat = {
                metric = {
                  metric_name = "RunningTaskCount"
                  namespace = "ECS/ContainerInsights"
                  dimensions = [
                    {
                      name = "ClusterName"
                      value = module.ecs_cluster.name
                    },
                    {
                      name = "ServiceName"
                      value = "queue-processing-service"
                    }
                  ]
                }
                stat = "Average"
              }
            },
            {
              id = "e1"
              label = "Calculate the backlog per group of instances"
              expression = "m1 / m2"
              return_data = true
            }]
          }
          target_value = 1000
      }
    }
  }

• I have executed pre-commit run -a on my pull request

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