25810: sql: Add the ability to access a specific column in a labeled tuple r=BramGruneir a=BramGruneir

This work is yet another step towards #16971.

The labeled tuples introduced in #25283 can now be accessed using their labels.
It's important to note that they require an extra parentheses around them to
do so.

```sql
SELECT (((1,'2',true) AS a, b, c)).a
+---+
| a |
+---+
| 1 |
+---+

SELECT (((1,'2',true) AS a, b, c)).b
+---+
| b |
+---+
| 2 |
+---+

SELECT (((1,'2',true) AS a, b, c)).c
+------+
|  c   |
+------+
| true |
+------+
```

This change prompted the need to restrict the labels in the tuple to be unique;
checks and test cases for that have been added as well.

Star expansion is still not allowed.

Release note: Labeled tuples can now be accessed using their labels (e.g.
`SELECT (((1,'2',true) AS a, b, c)).a` but it requires an extra level of
parentheses to do so.

26096: scripts: add Yahor to release notes r=yuzefovich a=yuzefovich

This adds Yahor Yuzefovich to the roachers list.

Release note: None

Co-authored-by: Bram Gruneir <bram@cockroachlabs.com>
Co-authored-by: yuzefovich <yahor@cockroachlabs.com>