Add support for AssumeRole credentials

[knorby]

Adds support to use AssumeRole granted credentials, using the either provided key or an instance profile.

My use case is with s3_file, so I can use a role on another account to gain access to a bucket. I'm less certain of the value for other resources, but I updated all of the resources anyway.

I tested it with s3_file and an instance profile, and it worked fine.

[chef-supermarket]

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