artifacts: Add Get endpoint

BUG=chromium:1034529
TEST=generate

Cq-Depend: chromium:2658727
Change-Id: Id4d538207656ee043ece9421326485ece3c78b65
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2660276
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
2 files changed