cas: enable compression
cas: enable compression

I saw some occationally slow uploads to RBE-CAS.
e.g. https://ci.chromium.org/p/chromium/builders/ci/Mac%20Builder%20Next/26049

Let me see whether compression can improve that.

Bug: 1157476
Change-Id: Ie14987817e8135d3e34e2770d8c9ec31091b7add
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2628816
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@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