Add a comment on pull request when a non collaborator submit a pull request

When a non collaborator user submit a pull request instead of being completely quiet about it we can add a comment letting know the user waiting for some collaborator to issue a /ok-to-test

an example of how prow message look like :

Hi username. Thanks for your PR.

I'm waiting for a [tektoncd](https://github.com/orgs/tektoncd/people) member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should [join the org](https://github.com/orgs/tektoncd/people) to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed [here](https://go.k8s.io/bot-commands?repo=tektoncd%2Fpipeline).

cc @sm43 :)

Start by tracing how pull requests from non-collaborators are detected and how automated testing status is reported. Implement the requested comment with the /ok-to-test guidance, then verify that it is posted for eligible pull requests without changing the behavior for collaborators.