Support substitution tokens in custom github notification context

Currently, when used within the context of a GiHub Branch Source hierarchy, all this plugin can do is let me set a custom but hardcoded notification context. (Since it is configured once for all jobs in the hierarchy)

It would be useful to be able to dynamically set a custom (different) context for all jobs in the whole branch source hierarchy. For example, I could resolve JENKINS-55726 when opening daggy fix PRs (same origin branch) by setting a custom context like this (hypothetical variable):

jenkins/${JOB_NAME}

Then I wouldn't have each job marking multiple PRs passed or failed. This is just one use case.

PR: https://github.com/jenkinsci/github-scm-trait-notification-context-plugin/pull/5