1. credential service may be accessed by callbacks
2. plan to add load_credential and save_credential method in CallbackContext (see cl/782158513) given customer has requirement to access credential service themselves. (see https://github.com/google/adk-python/issues/1816)
It's backward compatible given CallbackContext is parent class of ToolContext
PiperOrigin-RevId: 783480378
Xiang (Sean) ZhouandCopybara-Service