Refactor Ci::Pipeline's config_sources enum

What does this MR do?

This enum is now defined in the Ci::PipelineEnums module, allowing EE to extend this enum without having to modify Ci::Pipeline directly.

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/8244

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer