Fix Gitaly N+1 in /api/v4/project/:id/jobs (behind FF)

What does this MR do and why?

There's a gitaly N+1 in the jobs API. The commit field is retrieved via Commit.lazy, but only at serialization time, which forces the eager evaluation of each individual commit. We fix this by evaluating the commit method of each build in the paginated result set prior to serialization.

Given the criticality of the endpoint, this fix is behind the feature flag fix_jobs_api_gitaly_n_plus_1 (disabled by default), to derisk the rollout.

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Before	After

How to set up and validate locally

See the tests for an example.

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