Support for UNSIGNED-PAYLOAD

[dbolotin]

UNSIGNED-PAYLOAD as described here is not supported now.

If you want I can make a PR. But adding this functionality will require some API modification. I suggest the following changes:

• create sealed trait Payload with the following subtypes:
  • StringPayload
  • BytesPayload
  • PreCalculatedSHA256
  • UnsignedPayload (for UNSIGNED-PAYLOAD here)
  • StreamingPayload (for STREAMING-AWS4-HMAC-SHA256-PAYLOAD, as described here)
• change type of payload parameter from Option[Array[Byte]] to Payload

Please let me know your thought on this.

[dbolotin]

Please see my PR with less invasive modifications to support this type of requests.