Change all path extractors to use structs

[svix-andor]

This has 2 benefits:

1. It is a little safer because tuple params are extracted based simply on position so it's possible to make a mistake simply by putting the types in the wrong order. It still doesn't check that the path placeholders match the extractors at compile-time, though.
2. Aide can match field names to URL placeholder names this way, as a result including the path parameters in the OpenAPI params section.

[tasn]

TBH, it looks much better than the tuples!