Add more outputs to open-backport-pull-request

[yuya-takeyama]

This is useful when sending a notification based on the result of the action.
Slack notification example:

    steps:
      - id: backport
        name: Open Backport Pull Request
        uses: quipper/monorepo-deploy-actions/open-backport-pull-request@v1
        with:
          base-branch: ${{ env.BACKPORT_BASE_BRANCH }}
      - name: Notify to Slack
        if: ${{ startsWith(steps.open-pull-request.outputs.pull-request-url, 'https://') }}
        uses: Ilshidur/action-slack@2.1.0
        env:
          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK_URL }}
        with:
          args: |
            Backport ${{ steps.backport.outputs.head-branch }} into ${{ steps.backport.outputs.base-branch }}:
            ${{ steps.backport.outputs.pull-request-url }}

[chaspy]

LGTM 👍