feat(ai): support streaming (#899)

Author: atierian

packages/appsync-modelgen-plugin/src/__tests__/visitors/__snapshots__/appsync-model-introspection-visitor.test.ts.snap

@@ -326,7 +326,7 @@ exports[`Conversation Route Introspection Visitor Metadata snapshot should gener
                     \\"isRequired\\": false,
                     \\"name\\": \\"onCreateAssistantResponsePirateChat\\",
                     \\"type\\": {
-                        \\"model\\": \\"ConversationMessagePirateChat\\"
+                        \\"nonModel\\": \\"ConversationMessageStreamPart\\"
                     },
                     \\"arguments\\": {
                         \\"conversationId\\": {

packages/appsync-modelgen-plugin/src/utils/process-conversation.ts

@@ -185,7 +185,7 @@ function generateSubscriptionMetadata(routeName: string, modelName: string): Sch
     isArray: false,
     isRequired: false,
     name: `onCreateAssistantResponse${routeName}`,
-    type: { model: modelName },
+    type: { nonModel: 'ConversationMessageStreamPart' },
     arguments: {
       'conversationId': {
         name: 'conversationId',