Add trusted networks

[b4stien]

Deploying FastAPI in AWS behind an AWS Load Balancer I hit a roadblack to trust the proxy (the Load Balancer): I know its network but not its IPs. Using ProxyHeadersMiddleware as it is, I'd have to know those IPs beforehand to trust them.

What would you think of having ProxyHeadersMiddleware trusts some networks in addition to individuel IPs (as it's doing right now with trusted_hosts)?

I put some code of how it could work in #2387.