llama-server prompt cache support

[0x0539]

Does llama-server support prompt caching between requests, similar to llama-cli prompt file? I have a use case where the prefix stays the same between requests, with just a few characters changing at the end.

[0x0539]

Oh wait maybe that's on by default:

        params.cache_prompt     = json_value(data, "cache_prompt",       true);