Expose the raw webhook event JSON as a variable for pipeline parsing

[mtalexan]

What feature do you want to see added?

There have been significant changes to the GitLab Webhooks, and this gitlab plugin hasn't always been able to keep up, as the list of Issues about mismatches reveal. To make it more future-resistant, exposing the raw body of the webhook via a variable a pipeline can choose to consume would allow individual users to extract any data they need from a post-parsed triggering event, even if support for an added data field may not yet be present in the plugin.

Suitable for GSoC, as loosely discussed in https://github.com/jenkinsci/gitlab-plugin/issues/1005

Upstream changes

No response