Support passing bridge variables to downstream pipeline

What does this MR do?

Make it possible to pass bridge variables to a downstream pipeline. This merge request also adds support for using build metadata with CI/CD bridge jobs.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8997#note_136022455

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
• EE specific content should be in the top level /ee folder
• Security reports checked/validated by reviewer