Allow custom labels to be added

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**Is your feature request related to a problem? Please describe.**
I'd like to add some custom labels to my PRs. These labels can then be used to trigger other (external) automations. This is ofcourse possible using a separate github action (when using Github Actions), but it's easier if this would be a plugin for example.

**Describe the solution you'd like**
Just a simple plugin that can add a list of custom labels.

**Describe alternatives you've considered**
Using actions/labeler to add custom labels. This requires some extra logic, as you'd have to identify which PR was created by release please as a release PR. A plugin would just need some configuration, and almost no logic, maintained by the community.

**Additional context**
Add any other context or screenshots about the feature request here.