buffet: Support cloud command proxy. This proxy allows publishing command updates to cloud API. BUG=chromium:432540 TEST=cros_workon_make buffet --test&&manual Change-Id: Id5730df60c3d40dfb7254d5f65c84dc51aea09eb Reviewed-on: https://chromium-review.googlesource.com/228462 Reviewed-by: Anton Muhin <antonm@chromium.org> Commit-Queue: Anton Muhin <antonm@chromium.org> Tested-by: Anton Muhin <antonm@chromium.org>