`wasmtime-wasi[-http]`: support `implements` named imports

I am exploring using the new `implements` (aka "named imports") feature in a host. I have it working nicely with APIs that we control, but I have hit rocks with `wasi:http` (and will have the same problem with other WASI APIs). This happens because we use `wasmtime-wasi` and `wasmtime-wasi-http` for handling the WASI interfaces.

The trouble is that when, in my `wasmtime::component::bindgen`, I specify `named_imports: { "wasi:http/handler@0.3.0": SomeKey }`, this creates new `Host*` traits, with new types under the `named_imports` module. Not only is implementing all these host traits by hand is a chore, but it's laborious getting them to interoperate with `wasmtime-wasi-http` (and therefore with the rest of our HTTP configuration and stack), because they use different, newly bindgenned types, for things like `Request` and `Fields` and so on.

I am guessing it would be a lot less effort (and a lot less complexity) to modify `wasmtime-wasi` and `wasmtime-wasi-http` to handle `implements` of their underlying interfaces, but I'm not sufficiently familiar with the internals of these crates to tackle this myself or even offer a design suggestion. But I'm happy to provide more context or to try to dig in if that would be useful!