Introduce response header limits

[maxmoehl]

• Read the Contributing document.

Summary

• Fix the header accounting on requests.
• (internal) Rename the config field MaxHeaderBytes to MaxRequestHeaderBytes.
• Add MaxRequestHeaderBytes and the corresponding logic.

A request with too large response headers will now look like this:

$ curl -i "https://net-tools.cf.local/headers?foobarbazz=$(python3 -c 'print("a" * 1000)')"
HTTP/2 502
content-type: text/plain; charset=utf-8
x-cf-routererror: endpoint_failure (net/http: HTTP/1.x transport connection broken: net/http: server response headers exceeded 1024 bytes; aborted)
x-content-type-options: nosniff
date: Thu, 07 Nov 2024 09:24:09 GMT
content-length: 67

502 Bad Gateway: Registered endpoint failed to handle the request.

Backward Compatibility

Breaking Change? No

[maxmoehl]

This is the first set of changes, next up is limiting the number of headers as outlined in the issue. (and of course tests)

[maxmoehl]

I'm not sure whether we want to rename the gorouter yaml property from max_header_bytes to max_request_header_bytes or whether that's already considered breaking?

[maxmoehl]

Note to the reviewer: I've split the changes into review-able commits focusing on one aspect each so it's easier to review the individual changes :)

[peanball]

requesting to review some naming consistency and code comments for clarification (alignment with existing comment density)

[maxmoehl]

Once the routing-release PR is ready as well I will merge both.