Add tools/cmd/fakecas package
Add tools/cmd/fakecas package

It runs a fake CAS gRPC server for integration testing.
No change on remote-apis-sdks is required.

cas/swarming commands will support local CAS server in https://crrev.com/c/3205038

Bug: 1255534
Change-Id: I835e0fee50bcfe93095e6cc95b25cb08b94d21e8
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3205030
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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