Support assuming an IAM Role

[bilbof]

Hi, would it be acceptable for the s3-resource to support assuming an IAM role before authenticating to S3?

Our use case is that we have credentials (access_key_id and secret_access_key) for an unprivileged user that can assume a role which permits access to restricted S3 buckets.

This might involve adding an aws_role_arn field to the source configuration.

I think this is a slightly different request to concourse/concourse#3023, as we're not looking to use EC2 instance profiles. This approach is similar to the implementation in concourse/registry-image-resource

I made a first pass at this in alphagov@1b44254.

[taylorsilva]

With the way that commit implements it we'd be happy to merge a PR in for this feature.