feat: allow to backport unmerged prs

[tasso94]

This pull request is still a work in progress, but I wanted to share my first iteration to get feedback early on:

1. Whether it generally makes sense and if you want to merge it with your upstream.
2. If the technical design of the feature makes sense.

User story

As a user, I want to backport a PR that is still WIP and not merged with the master branch to allow running the CI on my maintenance branches.

Open tasks

I think the following topics still need to be addressed (is there even more I might not know yet?):

• [ ] Improve naming newly introduced config properties (I'm open to any ideas).
• [ ] Add tests.
• [ ] Write documentation.

[korthout]

Thanks @tasso94 ❤️

💭 It's important that backport-action doesn't automatically backport unmerged pull requests, as this could be unsafe in some repository workflows. But I do see that this could be useful to some as well. It should be a decision that the repository maintainer(s) can make.

👍 So I like the idea of adding this as an input, where the default behavior disallows backporting unmerged pull requests.

👍 I think the technical design makes sense.

🔧 We can clarify in the issue comment that the maintainer can use this input to allow unmerged pull requests.

[!IMPORTANT] Maintainers who want to enforce this input must avoid using the pull_request event to run the backport action. The pull_request event could allow the pull request author to flip the input flag before the pull request is approved and merged. The pull_request_target event prevents this. Note that backport-action promotes using the pull_request_target event in its readme.