Automatically set extraheader for both with and without ".git"

[sksat]

The gh-federation client uses Git's extraheader to automatically checkout the specified repository via gh-federation.
This configuration should specify the exact remote URL to be used for the actual checkout.

Therefore, the current implementation causes inconvenience in the following case.

- uses: arkedge/[email protected]
  with:
    endpoint: ${{ secrets.GH_FEDERATION_ENDPOINT }}
    repos: |
      <your-oug>/<something-internal-repository>

- uses: actions/checkout@v4
  with:
    # "https://github.com/<your-oug>/<something-internal-repository>.git" is in the .gitmodules
    submodules: recursive

In this example, the extraheader is set for the remote URL https://<your-oug>/<something-internal-repository>, but the actual checkout is set for https://<your-oug>/<something-internal-repository>.git, the checkout via gh-federation fails.

[sksat]

Alternatively, it may be better to simply add ".git" suffix when configuring extraheader. However, when implementing such an approach, it is necessary to verify the checkout without ".git" so as not to break the existing behavior.