Add pipeline name to job webhook event payload

What does this MR do and why?

Describe in detail what your merge request does and why.

Pipelines can now have custom names set in the .gitlab-ci.yml file (https://docs.gitlab.com/ee/ci/yaml/index.html#workflowname). This feature is currently behind the pipeline_name feature flag which is default enabled on gitlab.com and self-managed installations.

Add the pipeline name to the job webhook event payload since it makes it easier to identify which pipeline the job is part of. For example, if an action needs to be taken only for jobs of a particular pipeline, the pipeline name in the payload will allow the receiver to distinguish which pipeline the job belongs to, and accordingly perform the required action.

gitlab-com/gl-infra/delivery#2754 (closed)

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