[client][cas] specify client.Opts at NewClient
[client][cas] specify client.Opts at NewClient

CAS client options should be specified at initialization

Change-Id: I9ba50ee222d319e8462a0973ba080f9f126b4c06
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2588969
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>
Reviewed-by: Fumitoshi Ukai <0e88e065dd7f7d7f11da28b068df91c65342563c@google.com>

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