Add support for INCLUDE/EXCLUDE NULLS for UNPIVOT

[Vedin]

UNPIVOT structure is changed from 2023 and now supports INCLUDE or EXCLUDE NULLS.
Full syntax now looks like:

SELECT ...
FROM ...
    UNPIVOT [ { INCLUDE | EXCLUDE } NULLS ]
      ( <value_column>
        FOR <name_column> IN ( <column_list> ) )
[ ... ]

So, I added this functionality.

[iffyio]

I'm thinking it would be useful to include a description of this parameter - specifically highlighting that true means include nulls and false means exclude nulls (thinking otherwise at least the latter won't be obvious API wise to a user of the library without reading the code).

Also maybe we can call this property null_treatment similar to e.g. Function vs include_nulls which only covers one possible value naming wise?

[Vedin]

Yeah, agree. To not confuse it with null_treatment I add a new one null_inclussion. As I have checked it's used only in UNPIVOT.

[iffyio]

LGTM! Thanks @Vedin!
cc @alamb