add functions to download/upload from/to OSF

`osfr` is great but not too intuitive. I want to be able to do `download_from_osf('1xnb2', 'data/eyetracking/fixation-report.tsv')` and `upload_to_osf('1xnb2', 'data/prepped/fixations.Rds')`

- downloads to a file, uploads a file - no working with virtual files,
- if local file exists and its md5 is the same as reported by osf - don't download/upload,
- if there is no internet connection, fail gracefully,
- if permissions are needed, instruct to set up an OSF PAT

See 01_wrangling.Rmd in ht_seedlings for starter code.