PaC: Support default tasks in shared pipeline

When using a pipelinesascode.tekton.dev/pipeline annotation to reference a pipeline, we found that it is needed to import all the tasks used in that pipeline as well. This makes it very fragile, as any change in tasks in the referenced pipeline will break all builds using the pipeline. And also, this will make it very tedious to use. In our opinion, the tasks used in the referenced pipeline should be the responsibility of that pipeline, and that pipeline alone.

This would make PaC shared pipelines a lot more robust (default users cannot forget to import a task, or import wrong tasks, or updating the pipeline won't break all the pipelineruns), and it would make using them a lot more developer friendly. In my opinion it is a huge improvement if this would be implemented.