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

[ktrz]

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: #250 (comment)

[WorldSEnder]

I'm not sure how feasible it is to detect the PR that actually triggered an arbitrary workflow several layers deep. For example workflow_run could check the workflow that triggered it to recursively check for the PR it originated from.

So, a manual option would work for my use case.