feat(rt): mark event stream HTTP body as full duplex (#687)

Author: aajtodd

.changes/6313ed78-29f9-4d71-af08-38b6a62bcd40.json

@@ -0,0 +1,5 @@
+{
+    "id": "6313ed78-29f9-4d71-af08-38b6a62bcd40",
+    "type": "feature",
+    "description": "Mark event stream HTTP body as duplex stream"
+}

aws-runtime/protocols/aws-event-stream/common/src/aws/sdk/kotlin/runtime/protocol/eventstream/FrameEncoder.kt

@@ -44,6 +44,7 @@ public suspend fun Flow<ByteArray>.asEventStreamHttpBody(): HttpBody {
     return object : HttpBody.Streaming() {
         override val contentLength: Long? = null
         override val isReplayable: Boolean = false
+        override val isDuplex: Boolean = true
         override fun readFrom(): SdkByteReadChannel {
             // FIXME - delaying launch here until the channel is consumed from the HTTP engine is a hacky way
             //  of enforcing ordering to ensure the ExecutionContext is updated with the