Support EXTERNAL_QUERY tvf

Hi!

Is it possible to declare [EXTERNAL_QUERY](https://cloud.google.com/bigquery/docs/reference/standard-sql/federated_query_functions#external_query) tvf in ZetaSQL catalog? It is required to declare a return type that is a relation with known column names and types. In the `EXTERNAL_QUERY` we don't have this information and I couldn't find any way to declare tvf that returns table with unknown columns.