Cube API and Refresh Worker use Instance Metadata for AWS Credentials like Cube Store does

Right now it doesn't appear that the Cube API or Refresh Worker is capable of using AWS Instance Metadata to authenticate with AWS. However, the Cube Store containers already have this capability.

From a security and configuration management perspective, Role Based Access is greatly preferred over passing in long-term access credentials to the containers with environment variables. Is this already on the road map?

This would be very useful when using the `CUBEJS_DB_EXPORT_BUCKET` configuration.