SigV4 query-string URI-signing for AWS IoT Data service

[brianfaull]

I had need for a different query-string-based SigV4 signing for AWS IoTData service. The mechanism is quite different from the S3 or ES signing mechanism (already implemented in this library), so I wrote a new function to provide a SigV4 query string signing mechanism intended rather narrowly for the IoTData service for MQTT-over-WebSocket URIs, such as
wss://data.iot.us-east-1.amazonaws.com/mqtt
or account-specific endpoint like
wss://ABCD1234EFGH56.iot.us-east-1.amazonaws.com/mqtt
to produce output like
wss://ABCD1234EFGH56.iot.us-east-1.amazonaws.com/mqtt?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=...
Implemented in a separate file for convenience for now,
brianfaull@105b5fb
It uses a different signature than the S3-signing mechanism, handling URIs merely as string rather than embedded within http.Request for practical reasons.
I hope this is helpful. Feel free to incorporate it if you'd like. Alternatively, if you have requests for better integration and a proper pull-request, let's talk more.