Add options for labels

[simonhyll]

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• [x] Is this related to the actions-rs Actions? If you think it's a problem related to Github Actions in general, use GitHub Community forum instead: https://github.community
• [x] You've read the Contributing section about feature requests: https://github.com/actions-rs/.github/blob/master/CONTRIBUTING.md#feature-requests
• [ ] Is this something you can debug and fix? Send a pull request! Bug fixes and documentation fixes are welcome.

Motivation

When this action creates an issue it would be great if you could specify which labels it should add to that issue. I would personally use it to apply an "automated" label to it so I can filter it out from project boards.

In addition to manually specifying labels I would like labels for the severity. E.g. severity-unmaintained and severity-critical. This could be toggled on/off with a severityLabels bool option. It would also be nice to be able to edit those severity labels.

Workflow example

      - uses: rustsec/[email protected]
        with:
          token: ${{ secrets.GITHUB_TOKEN }}
          labels: ["automated", "audit"]
          severityLabels: true