Conditional `required` checks (#4544)

Workaround for skipped but `required` github checks. The idea is to
trigger the workflow but filter out unaffected jobs or steps. See [ci_cd
998](https://github.com/paritytech/ci_cd/issues/988) for details

In `.github/workflows/check-changed-files.yml` there is a reusable
workflow thad does all the checks and publishes results as outputs.
Example usage:
```
jobs:
  changes:
    permissions:
      pull-requests: read
    uses: ./.github/workflows/check-changed-files.yml
  some-job:
    needs: changes
    if: ${{ needs.changes.outputs.rust }}
 ....... 
 ```