Add variables on pipeline webhook

What does this MR do?

Add variables on pipeline webhook

Are there points in the code the reviewer needs to double check?

This is a pretty simple one liner

Why was this MR needed?

Pipeline variables are useful to create custom parametrised pipelines.

Variables can be used to track the purpose of a particular pipeline, and/or link to another higher level automatisation tool.

It is thus useful to get the list of variables in pipeline update events.

Variables are also not available in pipeline API: https://docs.gitlab.com/ee/api/pipelines.html#get-a-single-pipeline

There should be there as well, but this is another MR.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#28105 (closed)