more dynamic variables available from payload on PAC templates

this is to followup a request upstream here https://github.com/openshift-pipelines/pipelines-as-code/issues/396

quoting the query :

Four dynamic variables are available at the moment:

repo_owner
repo_name
repo_url
revision

When doing tasks like sonarscanner you need variables like pull request number, git base reference branch and git head reference branch in your pull_request.yaml (these parameters are stored in /tmp/payload.json)

It would be great if these variables also were available as dynamic variables. E.g.

pull_request_number
{pull_request_base_ref
{pull_request_head_ref

We need to take into account multiple VCS