Allow column unioning in Tables.columns

[bkamins]

Following the discussion in JuliaData/DataFrames.jl#3372 (comment) It would make sense to add cols kwarg to Tables.columns.

Let us discuss here if it would make sense, because maybe it is better to have a column-oriented equivalent of DictRowTable as a separate object.

CC @nalimilan @quinnj

[bkamins]

@nalimilan - I have realized that we have Tables.dictcolumntable to accompany Tables.dictrowtable so this issue can be closed. I will update DataFrames.jl accordingly.

[nalimilan]

Good. It would still make sense to allow opting into a stricter behavior than the default (which I find dangerous) using cols=:setequal or cols=:orderequal. But this is lower priority.

[bkamins]

Yes - You mean something like Tables.columns(table, cols=:orderequal)? @quinnj - is it clear what we have in mind here?