feat: Allow customizing the responses, flows in Captain (#11385)

- Ability to provide custom instructions to captain

<img width="1107" alt="Screenshot 2025-04-28 at 6 11 43 PM"
src="https://github.com/user-attachments/assets/f94cbccc-b4d8-48fd-b6b9-55524129bc50"
/>

app/javascript/dashboard/api/captain/assistant.js

@@ -14,6 +14,13 @@ class CaptainAssistant extends ApiClient {
       },
     });
   }
+
+  playground({ assistantId, messageContent, messageHistory }) {
+    return axios.post(`${this.url}/${assistantId}/playground`, {
+      message_content: messageContent,
+      message_history: messageHistory,
+    });
+  }
 }
 
 export default new CaptainAssistant();