api: structured outputs - chat endpoint (#7900)

Adds structured outputs to chat endpoint
---------

Co-authored-by: Michael Yang <mxyng@pm.me>
Co-authored-by: Hieu Nguyen <hieunguyen1053@outlook.com>

llama/sampling_ext.h

@@ -47,6 +47,8 @@ extern "C"
         llama_token id,
         bool apply_grammar);
 
+    int schema_to_grammar(const char *json_schema, char *grammar, size_t max_len);
+
 #ifdef __cplusplus
 }
 #endif