Enable mirroring on a project basis with individual authenticated http-proxy settings
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### 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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