DT-586: Support syncing private files from remote

[bkosborne]

Is your feature request related to a problem? Please describe.
A remote site uses private files. I want to sync these private files locally. The blt drupal:sync:files command does not sync private files, only public.

Describe the solution you'd like
A config option that includes private files when running blt drupal:sync:files, and/or an additional command like blt drupal:sync:files:private

[bkosborne]

I think drush may have support for this "private" variable already: drush-ops/drush#1113

[danepowell]

Makes sense. Not sure how people would use this most often, but I'm guessing it would make the most sense to have a sync.private-files config option. If it's set, then sync private files whenever drupal:sync:files happens.

This isn't a feature request we get often so it's not a high priority right now, but I'll put it on the roadmap. If you want to have it sooner PRs are welcome or you can advocate for it via a Support ticket.

[danepowell]

#3853 will add this functionality. Note that there's a potential bug in the currently implementation of local private files, which can cause conflicts if you are syncing multiple sites locally: #3854

Specifically, I think syncing a default site would wipe out any multisite private files locally.