Support private repositories and private submodules

Currently the checkout action doesn't work with private repositories using a private submodule.

As a work-around we use the following in our workflow.

steps:
      - name: clone main repository
        uses: actions/checkout@v2

      - name: clone submodule
        uses: actions/checkout@v2
        with:
          repository: our-organization/private-repo
          path: private-repo
          ref: v2
          ssh-key: ${{ secrets.SUBMODULE_SSH_KEY }}
          persist-credentials: false

It would be good if the checkout action would support some option to provide a different SSH_KEY for private submodules. E.g. SUBMODULE_SSH_KEY could be an organisation level SSH Key that allows pulling the repos.