[feature] add prompt prefix support for api mode.

[jiacheo]

For api mode, we don't need the ai to remember the whole context, but to add a prompt prefix for the after text.

Say: [prompt-prefix] translator all the later inputs into English: [user-input]: 今天天气不错 [ai-ouput]: The weather is nice today

if we had a [prompt prefix], we needn't input it every time, and we needn't care about the context tokens overflow.