feat: Add ability to register host functions with the runtime from the Rust SDK

[zshipko]

• Adds the ability to initialize plugins with additional functions created by the host (only supported by the Rust SDK right now)
• Adds PluginBuilder to the Rust SDK to make it easier to configure all the options

Hello, world

The following example uses println from the host to print to stdout from inside the plugin.

Host:

use extism::*;

fn main() -> Result<(), Error> {
    let context = Context::new();
    let manifest = Manifest::new([std::path::PathBuf::from("code.wasm")]);
    let say_hello = Function::new("say_hello", [ValType::I64], [], |caller, input, _out| {
        let internal = caller.data();
        let r = internal.memory().get_str(input[0].unwrap_i64() as usize)?;
        println!("Hello, {r}!");
        Ok(())
    });
    let mut plugin = PluginBuilder::new(manifest)
        .with_function(say_hello)
        .with_wasi(true)
        .build(&context)?;
    plugin.call("test", b"world").unwrap();
    Ok(())
}

Plugin:

use extism_pdk::*;

extern "C" {
    fn say_hello(offs: u64);
}

#[plugin_fn]
pub fn test(input: String) -> FnResult<()> {
    let memory = Memory::from_bytes(&input);
    unsafe {
        say_hello(memory.offset);
    }
    Ok(())
}

[nilslice]

Finally tested this and did my own functions / plugin calls - works great! Opens up so many cool options.

One thing that we will need to take into consideration (really just docs & awareness) is for the Playground - which will error out if a custom import is expected from a plug-in. I don't think there's really anything more we can do, bedsides maybe list those imports in the error and suggest that this plugin was made to use a custom host function.

I want to play more with this before we release it, and add some e2e testing around it. But it LGTM to merge 🤘

[zshipko]

Great! I was thinking we should just put out a very basic implementation and see what we can build around it before committing to something more complex. So I agree, let’s merge this and make additional PRs for any improvements.