Add full Event Stream marshalling support and working S3 example

[jdisanti]

Motivation and Context

This is more progress towards #121.

Description

• Completed implementation of the Event Stream marshall/unmarshall codegen
• Refactored Arc<Mutex<PropertyBag>> to be SharedPropertyBag
• Fixed bug where a signed empty frame wasn't being sent on end of stream (fixes BadRequestException response in the Transcribe example)
• Started on RPC initial-response support by adding try_recv_initial() to Receiver
• Reduced number of AWS services in tier-1 CI build

Testing

• Marshall/unmarshall codegen is thoroughly unit tested
• Manually tested against Transcribe StartStreamTranscription and S3 SelectObjectContent

Checklist

• I have updated the CHANGELOG

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[rcoh]

making this fields optional feels like a bit of a code smell, probably worth splitting it up to avoid running heterogenous data through this function.

[meta comment on cfbadaf]

[rcoh]

a few small cleanups. Only major thing IIRC was the generic error stuff. Exciting that this operations are working!

[rcoh]

this seems like the kind of runtime panic we should be very cautious about since it's not that hard to trigger. Feels like a level of TODO a bit higher than the others we should be very careful not to launch with

[jdisanti]

In the case where the target is the union member, it is required due to the nature of Smithy unions, so it's impossible to reach this unimplemented! in this case.

So that means this is only reachable for an event struct that has a @eventPayload annotated member that isn't also @required (so the builder isn't checking its requiredness). Will start looking into the expected behavior here.

For the models we are allow listing in this PR (Transcribe and S3), this isn't a problem since none of them need to serialize an event with a member annotated with @eventPayload. It's exclusively in the response events so far.

[rcoh]

LGTM! Excited to ship this!

[rcoh]

nice. We can customize this for S3 select to get nice errors at some point.

• open an issue to track

[jdisanti]

Issue opened: #671