text: Title: [Feature] Prefill assistant response
  Body: ### Checklist
  
  - [x] 1. If the issue you raised is not a feature but a question, please raise a discussion at https://github.com/sgl-project/sglang/discussions/new/choose Otherwise, it will be closed.
  - [x] 2. Please use English, otherwise it will be closed.
  
  ### Motivation
  
  OAI API doesn't natively support prefilling an assistants response. vLLM and Aphrodite has the additional support for `continue_final_message` which would be need to have for SGLang to give developers even much more control.
  
  Should be relatively easy for someone to implement. It's simply not allowing chat template EOS to take over in a turn where assistant response is last and this flag is enabled and a generation is requested. This was originally implemented with exact same parameter name in transformers, which became a feature in vLLM and Aphrodite.
  
  ### Related resources
  
  https://huggingface.co/docs/transformers/main/en/chat_templating
  https://github.com/aphrodite-engine/aphrodite-engine/blob/e64075b8937786311f6441fab5103f9ebf4e1dd8/aphrodite/endpoints/openai/protocol.py#L225-L233
  https://docs.vllm.ai/en/latest/serving/openai_compatible_server.html#id7
  
  Not seeing any extra parameter support
  https://docs.sglang.ai/backend/openai_api_completions.html
  