Create a folder for syncing labels

[RobertKielty]

@hh made the following comment on another issue

We'll need to create a folder for syncing labels: https://github.com/kubernetes/test-infra/tree/master/label_sync#label_sync

Let's look into creating a label_sync GitHub action job that get's trigged when the labels.yaml changes.

[RobertKielty]

/assign @RobertKielty