Support manually setting the relevant PR number that the action should comment to

If the action is not triggered by a pull_request trigger, in some cases it's impossible to infer what the relevant PR is. In those cases the easiest solution could be to have an input option to provide a PR number that we want the action to post to.

Relevant comment from another issue: https://github.com/benchmark-action/github-action-benchmark/issues/250#issuecomment-2307796766