cv: allow StageNewRunsFrom to return multiple creators.
cv: allow StageNewRunsFrom to return multiple creators.

A later change will make it return up to two creators: one for a new
patchset run, and one for a commit-queue label vote run (e.g. full run
or dry run).

R=yiwzhang

Bug: 1242951
Change-Id: I8d79e7f3f2275ed6bde97cf5603d2eaeee9fb7ab
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3817964
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Roberto Carrillo <b31161e510fbb37f8f70fdcde11530d2bae88470@chromium.org>

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