fix: Make tracing field optional

lambda-extension/src/events.rs

@@ -20,7 +20,7 @@ pub struct InvokeEvent {
     /// The function's Amazon Resource Name
     pub invoked_function_arn: String,
     /// The request tracing information
-    pub tracing: Tracing,
+    pub tracing: Option<Tracing>
 }
 
 /// Event received when a Lambda function shuts down.