(jenkins-jobs) Allow customization of the "Discover fork pull requests" trait

[dduportal]

What feature do you want to see added?

Code at https://github.com/jenkins-infra/helm-charts/blob/8800432356358003e7327ee6fbd337aad0193801/charts/jenkins-jobs/templates/_helpers.tpl#L213-L218 is:

• Not customizable (we want to disable PR from forks on some jobs on infra.ci.jenkins.io for instance)
• Tedious as it does not use the native JobDSL keyword (this code was written 2 years ago: worth checking if the JobDSL plugin has recent native support like https://github.com/jenkins-infra/helm-charts/issues/1211)

Upstream changes

No response