Refactor service classes

What does this MR do and why?

This MR refactors the service classes Ci::Runners::GetUsageByProjectService and Ci::Runners::GetUsageService (which share very similar implementations), in order to extract a base class which will make it easier to extend these classes in the future, e.g. for #440605 (closed).

It probably makes sense to review this MR commit-by-commit.

Part of #440605 (closed) and #463079 (closed)

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How to set up and validate locally

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There shouldn't be any changes in functionality, as shown by the unchanged tests.