Add support for determining upcoming maintenance

[clstokes]

Cloud providers indicate future maintenance is planned for instances. Expose this through aero.

• Amazon - https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeInstanceStatus.html
• Google - https://cloud.google.com/compute/docs/storing-retrieving-metadata#maintenanceevents

Separate instance-state vs maintenance-planned key?