Make aws-cloudwatch input to ensure at-least-once delivery

Filebeat guarantees that events will be delivered to the configured output at least once and with no data loss. Filebeat is able to achieve this behavior because it stores the delivery state of each event in the registry file. For `aws-cloudwatch` input, which is a part of Filebeat, we should also start using registry file and enhance it to ensure at-least-once delivery.

Start by locating the Go implementation of the aws-cloudwatch input and the existing registry-file handling; the issue names no files or tests. Trace how delivery state is tracked across retries and restarts, then identify tests needed to demonstrate at-least-once delivery and no data loss.

Written by the indexing model from the issue text.