Support AWS IAM roles for service accounts to access S3

WAL-G should be able to use the credentials:

WAL-G determines AWS credentials like other AWS tools. You can set AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY (optionally with AWS_SESSION_TOKEN), or ~/.aws/credentials (optionally with AWS_PROFILE), or you can set nothing to fetch credentials from the EC2 metadata service automatically.

See also:

https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html