Add AWS RDS (Relational Database Service) monitor

[calebhailey]

TODO:

• [ ] Init integration from template (see: https://github.com/sensu/catalog/blob/main/templates)
  • [ ] Add README.md
  • [ ] Add sensu-integration.yaml
  • [ ] Add CHANGELOG.md
• [ ] Add logo.png
• [ ] Add sensu-resources.yaml

Notes:

• Use the new https://github.com/sensu/sensu-cloudwatch-check plugin
• Configure output_metric_thresholds for alerting

[hillaryfraley]

I think this is closed by https://github.com/sensu/catalog/pull/258