trust_number_of_proxies to ProxyHeadersMiddleware

[mzealey]

I was needing to port code from https://werkzeug.palletsprojects.com/en/stable/middleware/proxy_fix/ where you can specify the (known) number of proxies in the chain, rather than their ips/ranges which may be unknown/changing. #2508 is now running successfully in a large production environment.

[mzealey]

Any updates on this? #2508 is really useful for us...