support STREAMING-UNSIGNED-PAYLOAD-TRAILER and STREAMING-AWS4-HMAC-SHA256-PAYLOAD-TRAILER

[amwolff]

As described in #89, we need to start supporting

• STREAMING-UNSIGNED-PAYLOAD-TRAILER
• STREAMING-AWS4-HMAC-SHA256-PAYLOAD-TRAILER

The first step of this issue is to figure out what it takes to implement the support in the current auth code. If it's going to be a major change that explodes in complexity, we need to gauge if it would be easier to implement a separate SigV2/SigV4 library that we would import in place of the current auth code.

[amwolff]

implementation reference: https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-streaming-trailers.html