client: make fake server more compatible to real server
client: make fake server more compatible to real server

* Store data as raw as possible in |contents| dictionary.
* Support content download for FAKE_GCS path.
* Apply base64 to binary in json resposen.
* Reply fake GCS path for larger file retrive request.

Bug: 1071740
Change-Id: I5b6182e018ad3de0bb261bc7db9e124e5d258fcb
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2157435
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py