Support setting arbitrary environment variables in foreman_plugin

[laugmanuel]

We should support setting arbitrary environment variables by passing them as an input to the job.
This would allow the caller/user to customize the test behaviour by reading the variables back in code.

See discussion in dm-drogeriemarkt/foreman_wreckingball#113

[laugmanuel]

@ekohl, @evgeni any feedback on the request?

[evgeni]

The request is valid, we just didn't have time to think about it further.

[laugmanuel]

@evgeni I've prepared a PR for this feature.
It looks a bit hacky as I didn't find another way of passing information from caller to the reusable workflow. Passing env directly is a limitation stated here: https://docs.github.com/en/actions/using-workflows/reusing-workflows#limitations