uploadstore: Support auth via EC2 role credentials
Created by: efritz
Fixes #31718 (closed). This PR adds the PRECISE_CODE_INTEL_UPLOAD_AWS_USE_EC2_ROLE_CREDENTIALS
environment variable to allow instances to use the EC2 metadata service to provide S3 credentials, rather than passing static access and secret keys into the pod.
Test plan
Will take AWS API at its word.