feat: add models for streaming messages

src/AnthropicClient/Models/MessageStartEventData.cs

@@ -0,0 +1,18 @@
+using System.Text.Json.Serialization;
+
+namespace AnthropicClient.Models;
+
+public class MessageStartEventData : EventData
+{
+  public ChatMessage Message { get; init; } = new();
+
+  [JsonConstructor]
+  internal MessageStartEventData() : base(EventType.MessageStart)
+  {
+  }
+
+  public MessageStartEventData(ChatMessage message) : base(EventType.MessageStart)
+  {
+    Message = message;
+  }
+}