Add GS offload params

Config.Output.gs_root_dir to specify directory to offload to
    (Inside the new submessage Output for future-proofing)
Request.Test.need_synchronous_offloads for whether it's needed at all
    This will be user-controlled, unlike the location

Change-Id: Icf2f345fc706dc7c8fbd82fc3ded76fceaa531ed
Bug: 1022986
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2020481
Tested-by: Jacob Kopczynski <jkop@chromium.org>
Reviewed-by: Alex Zamorzaev <zamorzaev@chromium.org>
Commit-Queue: Alex Zamorzaev <zamorzaev@chromium.org>
Auto-Submit: Jacob Kopczynski <jkop@chromium.org>
5 files changed