[bqschemaupdater] Add support for clustering
[bqschemaupdater] Add support for clustering

The motivation for this change is to cluster buildbucket table by
builder.project and builder.bucket fields, however
1) clustering currentely supports only top-level fields
2) clustering takes affect only when the amount of data is massive.
   Buildbucket tables are rather small in a given day.

It will be useful for test results.

R=a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org

Change-Id: I780f5e4ee9df89aded39634e75bc457f884c8a02
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1698682
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Auto-Submit: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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