Optimize ListConfigVariablesService (prefill variables service)

What does this MR do and why?

We recently added two changes to this service;

1. cd811a5a We started passing verify_project_sha to YamlProcessor to check the passed ref to avoid malicious sha's. (the ref parameter accepts a commit SHA as a value)

2. d273ad95 We started passing the ref name to YamlProcessor when fetching prefill variables. The reason was this; When using $CI_COMMIT_REF_NAME in include:rules, the correct jobs but the wrong prefilled variables were included because $CI_COMMIT_REF_NAME were not available.

For 1; we first check whether the passed ref parameter is actually a ref in the project. If so, we don't need to verify the calculated sha.

For 2; we don't have to use RefFinder because we already have ref.

Related to #468827

Expected result

I am expecting that the duration of Ci::ListConfigVariablesService will be decreased.

https://log.gprd.gitlab.net/app/r/s/71r3H

MR acceptance checklist

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