1. Proposed title of this feature request

Prevent PipelineRun Execution for Draft/WIP Pull Requests

2. What is the nature and description of the request?

Many SCM repositories support a "draft" state for proposed changes (ex: "Draft" pull request in GitHub).

Pipelines as Code currently executes PipelineRuns for any pull/change/merge request, as long as an appropriate matchers are configured (ex, on-event, on-cel-expression).

3. Why does the customer need this? (List the business requirements here)

Running pipelines incur compute cost, especially if a repository is capable of triggering multiple PipelineRuns in a single pull request.

In theory an `on-cel-expression` could support this, but it involves inspecting the GitHub body payload. This is very brittle, see https://pipelinesascode.com/docs/guide/matchingevents/#matching-pipelinerun-on-body-payload.

4. List any affected packages or components.

Pipelines as Code