Propose a new hook for mutating the query bounds

Hi hackers,

I am currently working on improving the cardinality estimation component in
PostgreSQL with machine learning. I came up with a solution that mutates
the bounds for different columns. For example, assume that we have a query

```
select * from test where X<10 and Y<20;
```

Our approach tries to learn the relation between X and Y. For example, if
we have a linear relation, Y=X+10. Then Y<20 is essentially equivalent to
X<10. Therefore we can mutate the Y<20 to Y<INT_MAX so that the selectivity
will be 1, and we will have a more accurate estimation.

It seems to me that we can achieve something similar by mutating the
pg_statistics, however, mutating the bounds is something more
straightforward to me and less expensive.

I am wondering if it is possible to have such an extension? Or if there is
a better solution to this? I have already implemented this stuff in a
private repository, and if this is something you like, I can further
propose the patch to the list.

Best regards,
Xiaozhe