feat: Add support for event stream initial-request to RPC-based protocols

[dayaffe]

Describe the feature

Initial messages are defined in the Smithy spec here: https://smithy.io/2.0/spec/streaming.html#initial-messages

Determine how initial request should be handled for RPC protocols and implement.

Depends on #1175

Use Case

To correctly handle initial request and make values easily available to stream consumers.

Proposed Solution

Once support for unbounded Event Stream payloads (RPC) is completed, initial-request can be supported by parsing initial request members (members in the request w/o @streaming trait) within the HttpMiddleware. In order to ensure that initail-request members are processed first by the intended service, these values should be added to a message with event-type 'initial-request' and pre-pended to the event stream before it is sent (as was done by the Kotlin SDK here).

Other Information

Implementing support for initial-request in RPC protocol-based event streams will first require adding support for unbounded event stream payloads (RPC). Currently, Event Streams (in request) in the Swift SDK are only supported over REST protocols where they are bound with @httpPayload.

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

[sichanyoo]

Effort is approximated assuming ticket #1175 is implemented.

[sichanyoo]

Quip doc with details: /e7tTALANYJ03/Event-Streams-Knowledge-Document

[jbelkins]

Shipped in AWS SDK for Swift 0.40.0