trust_number_of_proxies to ProxyHeadersMiddleware

[mzealey]

Summary

The motivation for this is if you know you are behind eg loadbalancer + reverse-proxy nginx, but their IPs are not sensibly known (eg they change), you can configure the middleware to just skip the known number of them before applying any other heuristics. This should be backwards-compatible with previous versions of the middleware. This makes it easier to port code from https://werkzeug.palletsprojects.com/en/stable/middleware/proxy_fix/

Adds a new setting of trusted_number_of_proxies to the
ProxyHeadersMiddleware that allows the user to specify the number of
proxies that are trusted to be present in the request and not otherwise
verified.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[mzealey]

(the tests currently seem to be failing due to the tests on master branch being broken?)

[Kludex]

Please create a discussion first.

[mzealey]

Opened in #2519