Static File Path Support

[captn3m0]

I would like to serve a robots.txt file. Having an option to set a static path directory would be the nicest way to do this - that is what I used to do on Gitea for eg.

[Vonfry]

One of implementation is that rgit supports X-Forwarded-For headers to append prefix with it, so reverse proxy can work correctly for rgit builtin static files and users create their own path router for others.