Im using the QuestionAnswerPrompt to generate responses based on our context - works brilliant. The template has two required variables (context_str and query_str), but appears locked to these two. I tried adding a third custom variabel, but cant. What are my options here - is it correct QuestionAnswerPrompt templates only work with the two (and only the two) variables? Assuming yes, how could I make a new template that ALSO uses the context_str?? Its not clear form the documentation.