refactor(metadata): generate all metadata in lua

Then we can just load metadata in C as a single msgpack blob. Which also
can be used directly as binarly data, instead of first unpacking all the
functions and ui_events metadata to immediately pack it again, which was
a bit of a silly walk (and one extra usecase of `msgpack_rpc_from_object`
which will get yak shaved in the next PR)

src/nvim/api/vim.c

@@ -1529,7 +1529,7 @@ Array nvim_get_api_info(uint64_t channel_id, Arena *arena)
 
   assert(channel_id <= INT64_MAX);
   ADD_C(rv, INTEGER_OBJ((int64_t)channel_id));
-  ADD_C(rv, DICTIONARY_OBJ(api_metadata()));
+  ADD_C(rv, api_metadata());
 
   return rv;
 }