add tar_resources_data.table

[smingerson]

Specifically, the threads/percent arguments to data.table::setDTthreads(). This will allow specifying the number of threads available for a target separate from the target's code. I'm open to other solutions that allow flexibility between different targets. Still learning all the differences between drake and targets, so might be missing something obvious.

[wlandau]

Are you thinking about this for tar_target(format = "fst_dt")? If so, you could control that with fst::threads_fst() in a project-level .Rprofile file. Same goes for tar_target(format = "feather") or "parquet", arrow lets you control the number of threads using functions at https://arrow.apache.org/docs/r/reference/index.html#section-configuration.