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

### What feature do you want to see added?

There have been significant changes to the [GitLab Webhooks](https://docs.gitlab.com/ee/user/project/integrations/webhook_events.html), 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_