Add feature flag to use SHA instead of persistent ref

What does this MR do and why?

Adds the beta feature flag runner_refspec_use_sha_instead_of_persistent_ref to use the pipeline SHA directly in runner refspecs instead of the persistent ref, refs/pipelines/:id. Fetching the SHA instead of the pipeline's persistent ref eliminates races related to the lifecycle of the ref's lifecycle (it gets deleted when the pipeline stops, re-created any time a build starts).

References

• Feature issue #591477 (closed)
• Epic gitlab-org#21097

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:runner_refspec_use_sha_instead_of_persistent_ref)

2. Set CI/CD variable GIT_TRACE to 1 in a project
3. Run a job, look for git fetch in the logs

Expected results:

• The first part of the fetch is the pipeline's SHA
• There should be no mention of refs/pipelines/:id

MR acceptance checklist

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