move marshaller and unmarshaler into internal pkg (#304) (#325)

internal/marshaller.go

@@ -0,0 +1,15 @@
+package openai
+
+import (
+	"encoding/json"
+)
+
+type Marshaller interface {
+	Marshal(value any) ([]byte, error)
+}
+
+type JSONMarshaller struct{}
+
+func (jm *JSONMarshaller) Marshal(value any) ([]byte, error) {
+	return json.Marshal(value)
+}