PAC: Add ability to start a pipelinerun manually

1. What is the nature and description of the request?
It is currently not possible to start a pipelinerun without requiring an event trigger. 
At the moment, you can only use  `/test <pipelinerun name>` when that pipelinerun has a `pull request` event. This means triggers on every commit, which is not always wanted for resource heavy and lengthy integration tests. 

2. Why does the customer need this? (List the business requirements here)
This makes it possible to have some pipelines that can be kicked off manually, without them running automatically on every commit. This unlocks a smoother way of working, and reduces unnecessary resource consumption, which is better for costs and the planet.

3. How would the customer like to achieve this? (List the functional requirements here)
The PR comment command `/test <pipelinerun name>`should be able to run pipelineruns that don't have a `pull request` event.

4. For each functional requirement listed in question 4, specify how Red Hat and the customer can test to confirm the requirement is successfully implemented.
We can add a pipeline without the  `pull request` event, and add the  `/test <pipelinerun name>`comment in a PR to see if it runs.