cas: add cas binary
cas: add cas binary

I will add subcommands later.

Bug: 1110569
Change-Id: Ieb63bf748448a22daa2ca5aabeea5a5d5fed4562
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2332136
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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