New Cloudwatch logs input plugin

@lukewaite has created a new input plugin for Cloudwatch logs that would be a great addition to the Logstash ecosystem. Let's bring this into the logstash-plugins repo.

https://github.com/lukewaite/logstash-input-cloudwatch-logs

Start by reviewing the existing plugin in the linked logstash-input-cloudwatch-logs repository and the contribution expectations in the logstash-plugins repository. Check how comparable input plugins are structured and tested. Done means the CloudWatch Logs input is integrated into logstash-plugins with the required packaging and tests.

Written by the indexing model from the issue text.