This commit adds a new "attachment" style resource for setting the
policy of an AWS S3 bucket. This is desirable such that the ARN of the
bucket can be referenced in an IAM Policy Document.
In addition, we now suppress diffs on the (now-computed) policy in the
S3 bucket for structurally equivalent policies, which prevents flapping
because of whitespace and map ordering changes made by the S3 endpoint.
James Nugent