Support different routing modes

[fibonacci1729]

Currently the Router routes requests using the incoming requests .path() which is absolute as opposed to matching against the matched suffix stripped of the route specified in the spin.toml. This has been reported as a source of confusion a few times in discord.

I think supporting both routing modes specified via configuration passed to the router constructor is a decent short term solution. Longer term we should probably rollout a deprecation of the ambiguous .handle(...) method on the router in favor of explicit methods for both modes (e.g. .handle_matching_suffix(...) and .handle_absolute(...) -- or whatever names).