sql: table-returning UDF loses declared column name when aliased in FROM

**Describe the problem**

A SQL-language UDF declared with `RETURNS TABLE (col ...)` loses its declared output column name when the function call is given a table alias in the `FROM` clause. Instead of keeping the declared column name, the single output column is renamed to the table alias itself.

This diverges from PostgreSQL, where a table alias renames the relation but leaves the column names intact (column names are only overridden by an explicit column alias list, e.g. `AS ul(x)`).

As a consequence, references of the form `alias.declared_column` fail to resolve, which breaks the common pattern of selecting a specific column out of an aliased table-function call.

**To Reproduce**

```sql
CREATE TABLE t (
id INT PRIMARY KEY,
v INT DEFAULT NULL
);

CREATE OR REPLACE FUNCTION f1()
RETURNS TABLE (out_col INT)
LANGUAGE SQL AS $$
SELECT v AS out_col FROM t LIMIT 2;
$$;

-- Correct: the output column is named `out_col`.
SELECT * FROM f1();
-- out_col
-- -----------
-- (0 rows)

-- BUG: the output column is named `f` (the alias) instead of `out_col`.
SELECT * FROM f1() AS f;
-- f
-- -----
-- (0 rows)

-- BUG (downstream consequence): fails with
-- column "f.out_col" does not exist
CREATE OR REPLACE FUNCTION f2()
RETURNS TABLE (out_col INT)
LANGUAGE SQL AS $$
SELECT f.out_col FROM f1() AS f;
$$;
```

**Expected behavior**

Consistent with PostgreSQL: a bare table alias (`AS f`) should rename only the relation, leaving the declared column name `out_col` intact. `SELECT * FROM f1() AS f` should return a column named `out_col`, and `f.out_col` should resolve. The column should only be renamed when an explicit column-alias list is supplied (e.g. `AS f(x)`).

**Environment:**
- CockroachDB version: reproduced on current `master`

**Additional context**

Discovered from a customer case where a table-returning UDF was invoked from another UDF using an alias; the wrapping function failed to create with `column "." does not exist`.

Related but distinct: #167213 covers dereferencing a table-valued function by its function name when *no* alias is given; this issue is about the case where an alias *is* given.

Jira issue: CRDB-67017

Start with the SQL reproduction in this issue and trace CockroachDB's handling of table aliases for SQL-language, table-returning UDFs. Compare bare aliases with explicit column-alias lists and verify that the declared column name remains available through the alias, including in the nested f2() example.

Written by the indexing model from the issue text.