Expose reverse proxy API for dynamic mounts

PUBLISHED_FROM=00772400bce7c15368d91741092ebc8ab0842e19

docs/c-api/http_server.h/mg_http_reverse_proxy.md

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+---
+title: "mg_http_reverse_proxy()"
+decl_name: "mg_http_reverse_proxy"
+symbol_kind: "func"
+signature: |
+  void mg_http_reverse_proxy(struct mg_connection *nc,
+                             const struct http_message *hm, struct mg_str mount,
+                             struct mg_str upstream);
+---
+
+Proxies a given request to a given upstream http server. The path prefix
+in `mount` will be stripped of the path requested to the upstream server,
+e.g. if mount is /api and upstream is http://localhost:8001/foo
+then an incoming request to /api/bar will cause a request to
+http://localhost:8001/foo/bar
+
+EXPERIMENTAL API. Please use http_serve_http + url_rewrites if a static
+mapping is good enough. 
+