Draft: Refactor external mapper verifier and file classes

What does this MR do and why?

This is the third step of #382531 (closed). In this MR, I am refactoring Gitlab::Ci::Config::External::Mapper::Verifier and the methods that it calls.

The main idea is that we are grouping files by their types and processing them. With this, we can implement batch requests and other performance improvements (finding the related project and sha and checking access only once).

The changes are behind a feature flag ci_refactoring_external_mapper_verifier (#386166 (closed)).

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