Add support for functional dependency for ROW_NUMBER window function.

[mustafasrepo]

Which issue does this PR close?

Closes #.

Rationale for this change

For window queries in the form ROW_NUMBER() OVER(PARTITION BY <empty> ORDER BY <expr>), we know that row numbers generated will consists of consecutive numbers across table. Hence result of this expression can be treated as PRIMARY_KEY. This enables us to execute queries in the form below

SELECT *
FROM (SELECT c1, c2, ROW_NUMBER() OVER() as rn
    FROM aggregate_test_100
    LIMIT 5)
GROUP BY rn
ORDER BY rn;

without this support query above should be re-written as

SELECT *
FROM (SELECT c1, c2, ROW_NUMBER() OVER() as rn
    FROM aggregate_test_100
    LIMIT 5)
GROUP BY rn, c1, c2 -- this line is the difference
ORDER BY rn;

to be able to execute.

What changes are included in this PR?

This PR adds ROW_NUMBER result as functional dependencies when it known that its result will be unique across rows (when partition by expression is empty).

Are these changes tested?

Yes

Are there any user-facing changes?

[ozankabak]

Simple and straightforward, LGTM

[ozankabak]

I will go ahead and merge this since it is a very simple change. In case I'm not seeing some possible issue related to this, we will promptly fix with a follow-on.