AWS SigV4A

[endgame]

S3 multi-region access points use a new type of signature algorithm, called SigV4A: https://shufflesharding.com/posts/aws-sigv4-and-sigv4a. We will have to support it at some point.

[endgame]

AWS have more details here: https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_sigv.html#how-sigv4a-works and some signing examples at https://github.com/aws-samples/sigv4a-signing-examples

This is used for multi-region requests like S3 multi-region access points or the CloudFront Key-Value Store. And we can't make requests at those services until we support endpoint-rule-set-1.json in #901.