Automatic merge from submit-queue.
Add sig labels and use aliases in OWNERS files
This PR does the following:
- Creates a sig-*-leads alias for all sigs and working groups
- Replaces the reviewers/approvers in the sig folders and design-proposals folders with that alias
- Adds a label field to the OWNERS files to allow for automatic sig labelling (https://github.com/kubernetes/test-infra/pull/3502)