Follow-up from "Draft: Implement reading inputs from various file sources in CI header specs"

The following discussion from !206931 (merged) should be addressed:

• @avielle started a discussion:

  We're now adding another responsibility to Interpolator, which already has a lot of responsibilities. Let's make a follow-up issue to explore refactoring how the Header is loaded and processed. A cleaner flow would be something like:

  Ci::Config → Process header includes → Yaml::Loader → Interpolator