[Metricbeat] Add AWS RDS Metricset

[kaiyan-sheng]

Amazon RDS Metrics: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/MonitoringOverview.html
https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/Aurora.Monitoring.html

Metricbeat Module / Dataset release checklist

This checklist is intended for Devs which create or update a module to make sure modules are consistent.

Modules

For a metricset to go GA, the following criterias should be met:

• Supported versions are documented
• Supported operating systems are documented (if applicable)
• Integration tests exist
• System tests exist
• Automated checks that all fields are documented
• Documentation
• Fields follow ECS and naming conventions
• Dashboards exists (if applicable)
• Kibana Home Tutorial (if applicable)

Metricbeat module

• Example data.json exists and an automated way to generate it exists (go test -data)
• Test environment in Docker exist for integration tests