Support variable in mod_header rule

[dblate]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] in NGINX, we can define add_header rule with variable like: location /webapp { add_header Access-Control-Allow-Origin $http_origin; }

but in BFE, it doesn't support variable like $http_origin now

Describe the solution you'd like A clear and concise description of what you want to happen.

support $http_origin and other common variables in header rule

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