Support approve github workflow

[liangyuanpeng]

Now, the label of ok-to-test only working for prow job, and this issue is talking about let ok-to-test also working for github workflow, it's mean always approve github action running when the PR have label of ok-to-test.

I remember that there was a corresponding discussion in the kubernetes/test-infra, but I can't find it now.

I think it can be a prow plugin.

/kind feature

[liangyuanpeng]

tracking at #194
/close

[k8s-ci-robot]

@liangyuanpeng: Closing this issue.

In response to this:

tracking at #194
/close

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