1. Proposed title of this feature request
Pipelines as Code: Parameter to receive a comma delimited list of changed files related to a pull_request or push event

2. What is the nature and description of the request?
Customer would like pipeline as code add a parameter with a comma delimited list of changed files related to a pull_request or push event similar to what is done in pull request (https://github.com/tektoncd/triggers/pull/1507) in the triggers repository.

3. Why does the customer need this? (List the business requirements here)
They have a pipeline that executes terraform. It needs to know which directory (or directories) to execute terraform in. They need a task to get the changed files from git/github. They would like to remove that task and use a PaC parameter with the comma delimited list of changes files.

4. List any affected packages or components.
NA