feat: Add shared_fn macro

[zshipko]

• Adds shared_fn macro to export functions with non-plugin-function signatures. All arguments will be treated as Extism pointers (u64)
• Adds extism_pdk::MemoryPointer<T> as part of the implementation of export_fn. It transparently wraps u64, making it possible to use it as an argument when writing export functions without any extism_pdk macros:

#[no_mangle]
pub unsafe extern "C" fn export_example(x: extism_pdk::MemoryPointer<String>)  {
    let s = x.get().unwrap();
    println!("Input string: {s}");
}

[bhelx]

Maybe ass an example of this here

[bhelx]

I'm a little unsure of name. The docs make it clear. But it's hard to tell which to use just from the name. I can't think of a better name though.

[nilslice]

Yea, agreed that although the docs make it clear, the name is a little ambiguous.

What about something like #[linkable_fn]? That is, a function which can be linked by Extism. I had also thought about #[shared_fn], since like a shared library, you don't execute it via a main entrypoint, but through some other execution path some code finds a function and calls it?

I think my main concern is that an Extism user, with even rudimentary knowledge of Wasm is going to see #[export_fn] and mistake it for #[plugin_fn], since the name correctly suggests the function will be exported.

[zshipko]

I like shared_fn - I will switch it