fix:fix open ai original validation. modify Tool's Function to pointer (#664)

Co-authored-by: caopengfei1 <caopengfei1@tal.com>

chat.go

@@ -226,7 +226,7 @@ const (
 
 type Tool struct {
 	Type     ToolType            `json:"type"`
-	Function FunctionDefinition `json:"function,omitempty"`
+	Function *FunctionDefinition `json:"function,omitempty"`
 }
 
 type ToolChoice struct {

examples/completion-with-tool/main.go

@@ -35,7 +35,7 @@ func main() {
 	}
 	t := openai.Tool{
 		Type:     openai.ToolTypeFunction,
-		Function: f,
+		Function: &f,
 	}
 
 	// simulate user asking a question that requires the function