Reading only a subset of columns

Please correct me if this is possible already. I looked through the source code and the documentation and did not find a clear way to do this: basically, I want to read a FeatherV2 file, but not mmap every single column. I already know which columns I need and I'd like to tell `Arrow.Table` the subset of columns I want read into memory.

This is similar to [this issue on Feather.jl](https://github.com/JuliaData/Feather.jl/issues/36).

This seems to be possible in the R arrow package using `col_select`.

Start at the Arrow.Table entry point for reading FeatherV2 files and review how column selection and memory mapping are handled. Compare the requested behavior with Feather.jl issue 36 and R arrow's col_select option; done means callers can select columns without mapping every column into memory.

Written by the indexing model from the issue text.