The use cases we want to cover is

1. By default, create a token scoped to the current repository
2. Allow to scope to a set of repositories by names for the current owner
3. Allow to scope to a different owner (all installation repositories)
4. Allow to scope to a set of repositories by names for a different owner

Our API idea is

      - uses: actions/create-github-app-token@v1
        id: app-token
        with:
          app_id: ${{ vars.APP_ID }}
          private_key: ${{ secrets.PRIVATE_KEY }}
          owner: some-owner
          repositories: some-repo1,some-repo2

Both owner and repositories would be optional, and one can be set to the other. owner defaults to the current repository owner, and repositories defaults to the current repository name.