Import stubs for native targets

Currently, `rust` bindgen inserts stubs for Wasm imports on native targets, which simply panic.

I'd like to discuss possibilities for making this configurable or perhaps define a convention of some sorts?

My exact use case is as follows:
At wasmCloud we've developed https://github.com/wasmCloud/wadge, which lets components be tested as native applications - for Go, it works by replacing each function with a `wasmimport` directive, generated by upstream bindgen, by an implementation, which calls an export on a component running in an embedded Wasmtime instance. E.g. https://github.com/wasmCloud/wadge/blob/c21abaf5487ad79dd17660858127b3232c2623f4/bindings/bindings.go#L22-L36 for `wasi:clocks/monotonic-clock@0.2.1#now`

This works by using [canonical ABI-*ish* ](https://github.com/wasmCloud/cabish) to read and write data, which is essentially just canonical ABI for native targets.

I'd like to be able to follow a similar approach for Rust components and in future - other languages, but Rust is the current focus.

A straightforward way to handle this could be configuring the stub generation for imports on native targets.

Perhaps `wit-bindgen` macro could take in a `NativeGenerator` or something similar as an optional argument, which would allow users to configure how these imports are generated?
Perhaps the API could look similar to https://github.com/bytecodealliance/wit-bindgen/blob/66bfda38bee488df27692311e364d79a3cd301a7/crates/rust/src/interface.rs#L457:
```rust
trait NativeGenerator {
fn generate_import(&mut self, func: &Function)
}
```

The trait could have a default implementation generating the `unreachable!` stub.

Of course there would need to be some API to facilitate the printing, perhaps `self` could expose the `src`, or the function might return a `String`

I'm happy to work on contributing this feature myself if there's an interest for getting it merged.