EE: Refactor Ci::Pipeline's config_sources enum

What does this MR do?

This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23799.

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.
• EE specific content should be in the top level /ee folder
• Security reports checked/validated by reviewer