Support joins on functions that return records

[fulghum]

User defined functions can return records, and Postgres allows joins on those results. Note that when a set returning function is invoked as a table function (e.g. SELECT * FROM my_func()), it returns the results as relational data, not as a set of records.

In Postgres, you can use the results from set returning functions to join on other data:

CREATE TABLE t (id INT PRIMARY KEY);
CREATE TABLE u (id INT PRIMARY KEY, val INT);

INSERT INTO t VALUES (1), (2);
INSERT INTO u VALUES (1, 10), (2, 20);

CREATE FUNCTION f(p_id INT)
RETURNS TABLE (id INT)
LANGUAGE SQL
AS $$
  SELECT p_id;
$$;

SELECT x.id, u.val
FROM t, f(t.id) AS x, u
WHERE u.id = x.id
ORDER BY u.val;

In PostgreSQL-15, this returns:

 id | val 
----+-----
  1 |  10
  2 |  20
(2 rows)

In the current Doltgres version, it returns this error:

ERROR:  table not found: t (errno 1146) (sqlstate HY000)

GMS provides a TableFunctionWrapper type that already wraps functions when they are used as a table function in a FROM clause. The main changes to enable joining on results of table functions are:

• Update the sql.Function interface (or another API) so it can report if a function is set-returning or not
• Update TableFunctionWrapper so that for set-returning functions, it unwraps the records
• Move the RecordValue type into GMS so that TableFunctionWrapper can access it
• Update the sql.Function interface (or another API) so that it can return it's schema