Add option to control start of response to generate api

[notasquid1938]

Using openwebui for ollama or textgenwebui. you can control what the model's response begins with to steer it in a certain direction. It would be very helpful to have this built-in to the api. I have struggled to recreate this effect with the api by trying to include the model's response portion of the template in my initial prompt but have been unsuccessful so far.