Provide github token via inputs instead of env. variables

[StefMa]

Right now you have to put the github token via environment variables:

    - uses: blackjacx/backlog-notifier@1.4.0
      with:
        repo-references: |
          {
            "data": [
              { "repo_id": "BACKLOG", "repo_name": "backlog" },
            ]
          }
      env:
        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

See

backlog-notifier/src/index.js

Line 17 in 15990ef

const token = process.env.GITHUB_TOKEN

If we move this to an input, then we can also provide the default github token.
Not sure if this is useful as probably this github token don't have access to other repos, but maybe you want to inform your issues within that repo as well? 🤔

Additionally, I think it looks nicer if this is part of the with block and doesn't require an extra env statement.