MDEV-24909 JSON functions don't respect KILL QUERY / max_statement_time limit

pass the pointer to thd->killed down to the json library,
check it while scanning,
use thd->check_killed() to generate the proper error message

include/json_lib.h

@@ -222,6 +222,7 @@ typedef struct st_json_engine_t
 
   int stack[JSON_DEPTH_LIMIT]; /* Keeps the stack of nested JSON structures. */
   int stack_p;                 /* The 'stack' pointer. */
+  volatile uchar *killed_ptr;
 } json_engine_t;