Cross-project pipeline triggers

What does this MR do?

This merge request implements first iteration of first-class cross-project pipeline triggers and exposes this feature to users.

It is hidden behind a feature flag.

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ee#8997

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• EE specific content should be in the top level /ee folder
• Security reports checked/validated by reviewer