lucicfg: Add crostesting common.proto to Starlark
lucicfg: Add crostesting common.proto to Starlark

BUG=chromium:956577
TEST=local run

Change-Id: I5c0746280abf279dca21f1d0d2fbea44124fd06e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1598202
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Sean Abraham <175a5718c779173dfba7a5f72889805f1af1d4f5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go