New `aws_cloudwatch_metrics` source

[binarylogic]

As part of https://github.com/timberio/vector-product/issues/59, we want to collect metrics from AWS Cloudwatch.

Prior Art

• https://github.com/influxdata/telegraf/tree/master/plugins/inputs/cloudwatch
• https://www.elastic.co/guide/en/beats/metricbeat/current/metricbeat-metricset-aws-cloudwatch.html#metricbeat-metricset-aws-cloudwatch

[jszwedko]

Linking RFC here: https://github.com/timberio/vector/pull/4179