[9.x] Support x-forwarded-prefix in TrustProxies.php

[ppodds]

#39996

If we want to deploy laravel project in a subfolder of the domain like the following Nginx setting. We need to use the x-forwarded-prefix header.

server {
  listen 80;
  listen [::]:80;

  server_name _;
  location /to/project/ {
    proxy_set_header X-Forwarded-Prefix /to/project;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $http_x_forwarded_proto;
    proxy_set_header X-Forwarded-Host my.domain;
    proxy_set_header X-Forwarded-Port 443;
    # laravel backend at localhost:8080
    proxy_pass http://localhost:8080/;
}

Symfony has already supported the x-forwarded-prefix header. We can deploy laravel in the subfolder of domain with this feature.