Implement `CDN-Loop` (RFC 8586)

RFC 8586 protects systems from intra-CDN infinite loops.

The basic requirement here would be to copy any cdn-loop header from an incoming HTTP request onto any outbound HTTP request(s).

Given that it is somewhat natural to implement "CDN-like" features in Spin, I think this should be an opt-out runtime-configurable feature.

Open questions around whether/how to expose this header to guests:

• Should it appear in inbound requests? I'd default to yes, but in some sense it may make the spin environment a little less portable.
• Should guests be able to add values? Seems fine to allow, in which case any outbound cdn-loop header value(s) must be sent after copied inbound value(s).
• If the answer to both of the above questions is "yes", we should probably be smart about only copying the header if the guest hasn't already (which it likely would if simply forwarding a request).

Start with RFC 8586 and the issue's proposed behavior for incoming and outbound HTTP requests. Trace where Spin handles guest requests and creates outbound requests, then resolve the open questions about guest visibility, guest-added values, ordering, and opt-out runtime configuration. Done means the behavior and configuration are specified and implemented with coverage for copied and guest-supplied header values.