perf: Refactored redcap_read to use future to allow for parallelization.

Created by: williamschmitt21

The commit in this pull request introduces some changes to redcap_read():

• The code now relies on purrr, furrr, future, and progressr.
• The calls to download the REDCap database in batches is now done using future_map to allow the end-user to compute in parallel with future::plan().
• progressr is used to temporarily display a progress bar to show download progress.
• Documentation is updated to reflect these additions.