Add transformation function to convert numeric to HLL/Sketch object

It would be handy if we have sth like the following:
```
select toHLL(user_id) as hll_user_id, toThetaSketch(user_id) as theta_user_id from T

hll_user_id | theta_user_id
|
```

`toHLL, toThetaSketch` will need some inputs for sketch configurations.