Add Aidan's decode method to get-account-id-from-keys

[Frichetten]

There has been a recent uptick in interest around decoding AWS account IDs from access keys. While we have a page on how to "officially" get an Account ID from keys using the API, I figured it would be a good time to add Aidan's decoding work. Both as a, "here is another way to do it", and as a means of ensuring that Aidan gets proper credit for his work.