[prpc] Add ServicerContext.clone
[prpc] Add ServicerContext.clone

Usage: https://chromium-review.googlesource.com/c/infra/infra/+/1114186

Bug: 854218
Change-Id: If9ae582f5b08d58ebc4ff8e598a49332213fffde
Reviewed-on: https://chromium-review.googlesource.com/1114242
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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