plpgsql: support accessing variables via `label.var_name` pattern

This works in Postgres:
```sql
DROP PROCEDURE IF EXISTS p;
CREATE PROCEDURE p() LANGUAGE PLpgSQL AS $$
BEGIN
<>
DECLARE
v text := 'test';
BEGIN
RAISE NOTICE '%', label.v;
END;
END;
$$;
```
Note that support for this access pattern makes sense the most when variable shadowing is allowed (#117508).

Relatedly, it's possible to access the input arguments via `routine-name.arg-name` pattern:
```sql
DROP PROCEDURE IF EXISTS p;
CREATE PROCEDURE p(param1 INT, param2 TEXT) LANGUAGE PLpgSQL AS $$
BEGIN
RAISE NOTICE '% %', p.param1, p.param2;
END;
$$;
CALL p(3, 'foo');
```

Jira issue: CRDB-37800