API builds improvements: ability to filter builds by stages
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=15532) </details> <!--IssueSummary end--> 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.
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