realip_remote_addr behavior easily leads to insecure code

https://docs.rs/actix-web/3.3.2/actix_web/dev/struct.ConnectionInfo.html#method.realip_remote_addr

This means that you have to use different functions depending on whether the application is behind a reverse proxy or not. But you tpyically do not want to recompile an application for something like this.

It would be much better if there was a config option indicating that the application is behind a proxy, and in that case have `remote_addr` return the correct one.

Realistically, there's a good chance that applications just use `realip_remote_addr` unconditionally, making it vulnerable to IP spoofing when it's not running behind a reverse proxy.

FWIW, the `host` logic seems to be similarly broken, since you can most likely pass a "bad" host header to an application if it's not behind a reverse proxy or that proxy does not filter out all the header this create uses.

FWIW, I recommend looking at how the [ProxyFix middleware for the werkzeug Python web framework](https://github.com/pallets/werkzeug/blob/main/src/werkzeug/middleware/proxy_fix.py) is implemented. It's opt-in, and lets you specify which proxy adds the data you want (in case of multiple proxies all adding to the headers).

Start with the linked ConnectionInfo::realip_remote_addr documentation and compare its behavior with remote_addr and host. Review Werkzeug's linked ProxyFix middleware for its opt-in proxy and trusted-proxy handling. Done should include a settled configuration/API direction, coverage for direct and proxied requests, and documentation that makes unsafe unconditional use clear.

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