Add CI/CD support for stage in via IAM roles

[jjacob7734]

Support MCP Long Term Access Keys via IAM roles as described at https://caas.gsfc.nasa.gov/display/GSD1/Using+Long-Term+Access+Keys. The key element is the role ARN templated as <arn:aws:iam::account-id:role/role-name>.

The summary of all the stage in use cases to be supported is at https://docs.google.com/presentation/d/1bTMKPRAeioxsKv7okcvWYjIbXoynmB8T8H2bL3vzYLU/edit#slide=id.g14535295eac_0_12

Definition of Done:

• [ ] Demonstrate role-based access to a MAAP HEC S3 bucket in the stage-in step.
• [ ] Documentation provided
• [ ] Github repo commits provided