llama-server: chat_format param

[taha-yassine]

Looking at the code from https://github.com/ggml-org/llama.cpp/blob/master/tools/server/server.cpp it seems there's a chat_format params that defaults to COMMON_CHAT_FORMAT_CONTENT_ONLY when not set. I couldn't find any reference to this in the docs, so I was wondering how it's supposed to be used.
@ochafik I think you're the one that introduced it so maybe you know the answer?
Thanks!