Audit when multi project down stream pipeline is created

What does this MR do and why?

Audit when multi project down stream pipeline is created

This commit adds audit event when a multi project downstream pipeline is created

EE: true Changelog: added

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Create 2 projects, group-1/project-A and group-1/project-B, and setup https://docs.gitlab.com/ee/administration/audit_event_streaming/#http-destinations in group-1

2. in project-A create gitlab-ci.yml with content:

stages:
  - trigger

trigger_project_b:
  stage: trigger
  trigger:
    project: "group-1/project-b" 
    branch: "main"

3. in project-B create gitlab-ci.yml with content:

stages:
  - build

build_job:
  stage: build
  script:
    - echo "Running build job in Project B..."
    - sleep 5

4. Run pipeline in project-A.
5. Check audit event streamed contains correct attributes.

Related to #481325 (closed)