[cas] change flag for dump digest
[cas] change flag for dump digest

I think `<hash>/<size>` format is more easy to treat in CLI interface.

Also this aligns the behavior of isolated client.
https://source.chromium.org/chromium/infra/infra/+/master:go/src/go.chromium.org/luci/client/cmd/isolated/lib/archive.go;l=60;drc=1a5bec19c988fab6ed9cc90b3fdcd18e800ea388

Bug: 1110569
Change-Id: Ibbdea909d23d65022692e0b8ec78a273960097bc
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2395936
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Reviewed-by: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
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