Implement Max-Forwards logic

Max-Forwards is an http header that can be used to limit the number of hops a request can go through. YARP should decrement this header if it sees it and block the request if the counter reaches 0.

See [RFC 7231](https://datatracker.ietf.org/doc/html/rfc7231#section-5.1.2)

The issue names no files or tests; start by reading RFC 7231 §5.1.2 and locating YARP’s request-forwarding path. Done means requests with Max-Forwards are decremented, and requests whose counter reaches zero are blocked, with behavior covered by appropriate tests.

Written by the indexing model from the issue text.