In support of the signing rework we're going to want to have
the signers keep track of the calculated hash to ensure that
we don't have a TOCTOU type issue. This commit adds the stat
command to the gsutil module so that we can ask gs a files hash.

Change-Id: Ib35d1541292a18fbfc26fab50f594c682ba4109d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1754262Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: George Engelbrecht <engeg@google.com>