Enable mirroring on a project basis with individual authenticated http-proxy settings
<!--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=25755) </details> <!--IssueSummary end--> ### Problem to solve Currently, there is no way to have repository mirroring behind a HTTP-Proxy with username/password authentication per project. This currently is an application-wide change and cannot be changed per project. HTTP-Proxy with username/password authentication will be implemented in `11.6` https://gitlab.com/gitlab-org/gitlab-ce/issues/44573 however it would be also useful to be able to set this up per project basis. ### Links / references ZD: https://gitlab.zendesk.com/agent/tickets/110119
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