forwarded_allow_ips should allow wildcard IP addresses

[Shiakaron]

In the use case of kubernetes, where the reverse proxy pods can have any IP address looking like 192.168.*.*. Therefore it would make sense to allow adding IP addresses with wildcards in the forwarded_allow_ips list, instead of specifying * to trust everything, or passing in a list of 254^2 (~65000) addresses.

All that would be required is a little tweaking in the ProxyHeadersMiddleware.get_trusted_client_host method

[toofishes]

After some recent changes (#2468), you can use network specifiers as well, after looking at the code: https://github.com/encode/uvicorn/blob/master/uvicorn/middleware/proxy_headers.py#L67-L75

So something like FORWARDED_ALLOW_IPS=192.168.0.0/16 would work for your example.