SigV4 Event Streams don't support initial events #4397
Description
EventStream try_recv_initial_request today doesn't work when the event stream has a SigV4 envelope. Because the initial message is wrapped in a frame, it can't see inside it to see that the the actual event type inside the frame is initial-requst
This is because try_recv_initial is not aware of the SigV4 framing.
The fix is to incorporate try_recv into the Unmarshall message trait so that the SigV4 unmarshaller can unwrap the message from the envelope properly. There is definitely some trickienss here though since the server does also need to gain access to the initial signature so that the signature can be validated.