sql: RETURNS TABLE not supported for UDFs

[rytaft]

Describe the problem

Trying to create a UDF with RETURNS TABLE currently results in a syntax error.

To Reproduce

Run the following in cockroach demo on master / 23.1:

CREATE FUNCTION sum_n_product_with_tab (x int)
RETURNS TABLE(sum int, product int) AS $$
    SELECT $1 + tab.y, $1 * tab.y FROM tab;
$$ LANGUAGE SQL;

On CRDB it results in this error:

invalid syntax: statement ignored: at or near "table": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
CREATE FUNCTION sum_n_product_with_tab (x int)
RETURNS TABLE(sum int, product int) AS $$
        ^
HINT: try \h CREATE FUNCTION

Expected behavior

This works on Postgres, so we should support it. But in the meantime, at least we should return an unimplemented error with an appropriate issue number.

Jira issue: CRDB-26307

gz#20015

[rytaft]

@mgartner I put this in 23.1 since I'm not sure if you care about telemetry for this feature? If not, feel free to move this to backlog.

[giangpham712]

@fqazi This affects efcore.pg tests