Add more dynamic variables

[skv-patrik]

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}}

[chmouel]

@skv-patrik the idea at first is that only the variable that are common to pull and push is exposed, we only expose the absolute necessary because we dont want to "diverge" too much from tekton/kubernetes templates.

I am not against it, and I did find myself in need of them, so I think those could be exposed when we are doing a pull request.

we support other type of VCS as well and those needs to be mapped properly.

[chmouel]

I have added an issue in our backlog https://issues.redhat.com/browse/SRVKP-1946

[chmouel]

{{{pull_request_base_ref}}
{{{pull_request_head_ref}}

was already added as source_branch/target_branch as per https://pipelinesascode.com/docs/guide/autoringprs/

just adding pull request number in #541