feat: support custom SSH user in git auth configuration with submodules

[axi92]

I have tested it on our instance of GHEC and it works.
We are using an github installation app token to checkout submoduls:

That was the tested workflow:

name: Checkout
on: push
permissions:
  contents: read
jobs:
  submodule:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/create-github-app-token@bcd2ba49218906704ab6c1aa796996da409d3eb1 # v3.2.0
      id: app-token
      with:
        owner: ORG
        app-id: 9999
        private-key: ${{ secrets.GLOBAL_APP_PRIVATE_KEY }}
    - uses: axi92/checkout@ed390c16724bd8db7b45b9670377a72a34a99d76
      with:
        ref: ${{ github.head_ref }}
        token: ${{ steps.app-token.outputs.token }}
        ssh-user: 'custom'
        submodules: true

• Allow sshUser setting to override default 'git' user in insteadOf config
• When sshUser is empty, default to 'git@github.com'
• When sshUser is provided (e.g., 'customuser'), use 'customuser@github.com'
• Updated tests to verify custom SSH user behavior in auth configuration

This resolves #2417