[aws-cloudwatch-metrics] Add support for mounting prometheus.yaml from ConfigMap

[tvandinther]

Is your feature request related to a problem?
I would like to configure the aws-cloudwatch-metrics helm deployment to mount a prometheus.yaml from a ConfigMap as per some of the tutorials for using the CloudWatch Agent for Prometheus scraping.

Is your feature request related to a specific Helm chart, if yes mention name of the chart?
aws-cloudwatch-metrics

Describe the solution you'd like
A value added to the chart to support this configuration.

Describe alternatives you've considered
The alternative we are currently going for is deploying all of the manifests manually where we can edit the volume mount of the deployment directly.