api/msgpack-rpc: Refactor metadata object construction

Instead of building all metadata from msgpack-gen.lua, we now merge the
generated part with manual information(such as types and features). The metadata
is accessible through the api method `vim_get_api_info`.

This was done to simplify the generator while also increasing flexibility(by
being able to add more metadata)

src/nvim/api/vim.c

@@ -528,10 +528,15 @@ void vim_register_provider(uint64_t channel_id, String feature, Error *err)
   }
 }
 
-/// Returns a feature->method list dictionary for all pluggable features
-Dictionary vim_discover_features(void)
+Array vim_get_api_info(uint64_t channel_id)
 {
-  return provider_get_all();
+  Array rv = ARRAY_DICT_INIT;
+
+  assert(channel_id <= INT64_MAX);
+  ADD(rv, INTEGER_OBJ((int64_t)channel_id));
+  ADD(rv, DICTIONARY_OBJ(api_metadata()));
+
+  return rv;
 }
 
 /// Writes a message to vim output or error buffer. The string is split