plpgsql: implement variable shadowing

In postgres, it is possible to shadow variables or parameters by declaring a variable with the same name in an inner block. This issue tracks adding support for variable shadowing. Ex:
```
CREATE PROCEDURE p_shadow(x INT) LANGUAGE PLPGSQL AS $$
DECLARE
x INT := 5;
BEGIN
RAISE NOTICE '%', x;
END
$$;

CALL p_shadow(100); -- Should print 5 instead of 100.
```

Jira issue: CRDB-35222