Automatically add a label to the PRs opened by all-contributors

[GyulyVGC]

Describe the solution you'd like
I would like that the PRs opened by the bot was automatically labeled with a label of choice.

Describe alternatives you've considered
This could be achieved adding a label field to the all-contributors JSON configuration file.

Additional context
I tried to search in the docs and in the past issues and I didn't find an option to do that, I hope to have not missed it!

[Berkmann18]

That's a sound feature to have; there's no config for this at the moment, I'll suggest opening a PR (especially as you likely have or had an idea of how to accomplish this).