Support arbitrary return type for custom SQL functions

According to [Hasura documentation](https://hasura.io/docs/1.0/graphql/manual/schema/custom-functions.html#creating-exposing-sql-functions), only `SETOF ` is a valid return type for Hasura to be able to track custom functions. A helpful note recommends creating an empty table as a workaround. But this imposes a lot of extra development and schema management.

Would it be easy to support custom / on-the-fly table definitions? For example:

```
CREATE OR REPLACE FUNCTION test_function()
RETURNS TABLE(intFirst INT, bSecond BOOLEAN, randomData JSONB) AS $$

SELECT 1, TRUE, '{"fake": "data"}'::JSONB;

$$ LANGUAGE SQL STABLE;
```