API for global push rules
<!--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=355880) </details> <!--IssueSummary end--> There is currently a way to manage [project](https://docs.gitlab.com/ee/api/projects.html#push-rules) and [group](https://docs.gitlab.com/ee/api/groups.html#push-rules) push rules using the API, but not global ones. Global push rules should have API support, just like [project push rules](https://docs.gitlab.com/ee/api/projects.html#push-rules) and [group push rules](https://docs.gitlab.com/ee/api/groups.html#push-rules). This was brought to our attention [by a customer](https://gitlab.zendesk.com/agent/tickets/274742) (internal link).
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