Add support for Amazon CloudWatch Logs as an Audit Store

This change is to add CloudWatch Logs to the list of Ranger supported audit stores. With this change, Ranger users will be allowed to configure their plugins to send audit events to Amazon CloudWatch Logs. Further, customers can query the events using Amazon CloudWatch Insights.

This functionality is built with a newly introduced audit destination 'AmazonCloudWatchAuditDestination'. Ranger users can enable it in the way similar to other types of audit destinations like Solr.