Add authorization configuration for requests to S3

[johakoch]

Requests to S3 buckets need a special sort of authorization: https://docs.aws.amazon.com/AmazonS3/latest/userguide/RESTAuthentication.html

Authorization = "AWS" + " " + AWSAccessKeyId + ":" + Signature;

Signature = Base64( HMAC-SHA1( UTF-8-Encoding-Of(YourSecretAccessKey), UTF-8-Encoding-Of( StringToSign ) ) );

StringToSign = HTTP-Verb + "\n" +
	Content-MD5 + "\n" +
	Content-Type + "\n" +
	Date + "\n" +
	CanonicalizedAmzHeaders +
	CanonicalizedResource;

CanonicalizedResource = [ "/" + Bucket ] +
	<HTTP-Request-URI, from the protocol name up to the query string> +
	[ subresource, if present. For example "?acl", "?location", or "?logging"];

CanonicalizedAmzHeaders = <described below>

Maybe, this could be configured as a child block of backend:

backend "..." {
  origin = "..."

  s3_signature_v2 {
    access_key_id = "..."
    secret_access_key = "..."
  }
}

Low-level alternative: Implement an hmac() HCL function:

	mac := hmac.New(sha1.New, secret_access_key)
	mac.Write(string_to_sign)
	sum := mac.Sum(nil)

(base64_encode() is already there.)