This PR adds methods `recommendForUserSubset` and `recommendForItemSubset` to `ALSModel`. These allow recommending for a specified set of user / item ids rather than for every user / item (as in the `recommendForAllX` methods).

The subset methods take a `DataFrame` as input, containing ids in the column specified by the param `userCol` or `itemCol`. The model will generate recommendations for each _unique_ id in this input dataframe.

## How was this patch tested?
New unit tests in `ALSSuite` and Python doctests in `ALS`. Ran updated examples locally.

Author: Nick Pentreath <nickp@za.ibm.com>

Closes #18748 from MLnick/als-recommend-df.