Changes:
- add the "wrapper" parameter to luci.recipe()
- add shadow bucket

Bug:1339006
Change-Id: I0f5c23f631b23b46fb40c728c432fb759bcefd7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3736695
Commit-Queue: Yulan Lin <yulanlin@google.com>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>