Add an interpolated flag to Yaml::Result

What does this MR do and why?

This MR adds an interpolated flag to Yaml::Result. We use this flag to determine whether or not to increment the interpolation counter in External::File::Base, and it can be used in the future to control interpolation side effects in other uses of Ci::Config::Yaml::Loader.

This allows us to clean up Yaml::Loader and Yaml::Interpolator by removing the current_user argument from both.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #415802 (closed)