fix(json-rpc): merge initial-response event into operation response

[kuhe]

This adds an eventStream serde helper for checking the first event of an event stream.

• if the type is initial-response, then it is merged into the operation output.
• if the type is not initial-response, then it is re-emplaced onto the iterator given to the user.

Additionally, for eventStream deser functions, deserialization of the implicit initial-response event is added to the union to avoid ignoring that unmodeled union member.

Note: this behavior is specific to AWS. It should have no effect on JSON eventstreams that do not implement initial-response, but there are also no hooks available to override this behavior in AWS SDK TypeScript codegen at the moment.

If non-AWS users want to have members other than the streaming member for the output, this could also be of use to them.

[kuhe]

diff example for JSv3

modified code for command deser (streaming)

/**
 * deserializeAws_json1_1XYZCommand
 */
export const de_XYZCommand = async (
  output: __HttpResponse,
  context: __SerdeContext & __EventStreamSerdeContext
): Promise<XYZCommandOutput> => {
  if (output.statusCode >= 300) {
    return de_XYZCommandError(output, context);
  }
  const contents = await bufferInitialResponse("streamingMember", de_XYZResponseStream, output, context);
  const response: XYZCommandOutput = {
    $metadata: deserializeMetadata(output),
    ...contents,
  };
  return response;
};

additional code in streaming member deser (de_XYZResponseStream)

    if (event["initial-response"] != null) {
      const ir = await parseBody(event["initial-response"].body, context);
      return {
        "initial-response": map({
           someNonStreamingMember: [, ir.someNonStreamingMember],
        }),
      } as any;
    }

in the above example, someNonStreamingMember is merged into contents which also contains the primary streaming member. Currently, only the streaming member is populated in a streaming operation response structure.