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

internal/unmarshaler.go

@@ -0,0 +1,15 @@
+package openai
+
+import (
+	"encoding/json"
+)
+
+type Unmarshaler interface {
+	Unmarshal(data []byte, v any) error
+}
+
+type JSONUnmarshaler struct{}
+
+func (jm *JSONUnmarshaler) Unmarshal(data []byte, v any) error {
+	return json.Unmarshal(data, v)
+}