[SNEAKER-59] Optimize `sneaker::json::json_schema_internal::get_ref`

`sneaker::json::json_schema_internal::get_ref` is a private function for parsing the value of a `$ref` value defined in a JSON schema. The function works by splitting the value string, read the first chunk, and recursively calls itself with the rest of the chunks putting back into string format.

While this currently works fine, it's not ideal performance wise. The repeatedly string splitting and joining could be avoided.