API builds improvements: ability to filter builds by stages

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For now it is possible to filter builds in project GET /projects/:id/builds only by scope

The scope of builds to show, one or array of: pending, running, failed, success, canceled; showing all builds if none provided

It will be good to filter also by stage. As an example - often you want to get last successful/failed build for a particular stage. Like give me last success run for unit-tests stage for project MyProject.