Support AWS `application/vnd.amazon.eventstream` event stream encoding

[endgame]

https://smithy.io/2.0/aws/amazon-eventstream.html

This is a format used by a few AWS APIs (e.g. s3:SelectObjectContent.

S3 sets "eventstream": true on the SelectObjectContentEventStream shape, but it's also used by a few other services (e.g. lexv2-runtime).

C implementation: https://github.com/awslabs/aws-c-event-stream

Discovered while investigating #965

[endgame]

@nitinprakash96 you may want to subscribe to this.