Poll GitHub API instead of using webhooks

This project sounds very interesting for the needs of my lab. Unfortunately, opening a port plus having a reverse-proxy running on a machine that submits SLURM jobs is not really an option for me. As an alternative to using a webhook, would it be possible to poll the GitHub API instead?

I dug a little bit, sounds like you [get the worflows of a repository](https://docs.github.com/en/rest/actions/workflows?apiVersion=2022-11-28#list-repository-workflows) with:

```
https://api.github.com/repos/USER/NAME/actions/workflows
```

then you [get the latest runs of a workflow](https://docs.github.com/en/rest/actions/workflow-runs?apiVersion=2022-11-28#list-workflow-runs-for-a-repository) with:

```
https://api.github.com/repos/USER/NAME/actions/workflows/WORKFLOW_ID/runs
```

filter the queuing runs, then you [get the jobs of a run](https://docs.github.com/en/rest/actions/workflow-jobs?apiVersion=2022-11-28#list-jobs-for-a-workflow-run) with:

```
https://api.github.com/repos/USER/NAME/actions/runs/RUN_ID/jobs
```

filter the self-hosted jobs and you get the expected labels for the runner to spawn.

I'd love to contribute to this, but I'm not familiar with Rust.