[bb] Allow bb to take arbitrary fields as field mask
[bb] Allow bb to take arbitrary fields as field mask


R=nodir, tandrii

Bug: 1039823
Change-Id: I4c607bce98f6b7b61728501ae73effb361287f3f
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2036764
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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