Support canary deployments with APIG sub stages

[HyperBrain]

AWS now supports sub stages for APIG stages, that let you do canary deployments easily. Additionally, the canary stage can "take over" the requests incrementally until it is fully serving the API requests.

This feature would complement the plugin, so that AWS CodeDeploy is not needed at all to provide the same canary deployments, that they provide.

Preliminary specification:

The plugin could offer a new --canary switch that would deploy the selected alias as a canary instead of replacing the already deployed version. So the deployment can be easily tested without breaking the "old" deployed alias stage. However, it has to be discussed how to set the canary configuration, as there are multiple configurations available from AWS.