[Tools] Add Spanner Data Updater tool.
[Tools] Add Spanner Data Updater tool.

Write a command to do a partitioned DML update with a longer
timeout than possible through the gcloud command.

BUG=1243174
TEST=Tested long-running update on chops-spanner-dev.

Change-Id: I34f9b63a77464894c40dda64ac95b4d626095f98
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3702880
Reviewed-by: Matthew Warton <5442985bfd1a148775701611fc78ffe955b8addb@google.com>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Patrick Meiring <496ae7b7898d641db15cbb7f3340e9390316ae10@google.com>

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