Add new contributor under the current pull request

**Is your feature request related to a problem? Please describe.**
When asking the bot to add a new contributor in a comment under a pull request, it'll open a new pull request. This is unnecessary, and it'd be better to add the contributor's profile right there under the PR, i.e. together with the actual contribution.

If there's a CLA bot, it'll also cause issues by requesting the allcontributors bot to accept a CLA (like [here](https://github.com/OrchardCMS/OrchardCore/pull/15567), though the CLA bot's comment vanished for some reason).

**Describe the solution you'd like**
Add the contributor as a new commit under the current pull request.

**Describe alternatives you've considered**
We usually close the new PR and merge its branch into the original contribution's branch. This works, but it's a chore.

**Additional context**
I don't have any more.