feat: add allowed_paths to specify preopened directories in WASI

[zshipko]

Adds ability to make local files available to plugins. Using the Rust SDK:

let wasm = include_bytes!("code.wasm");
let mut context = Context::new();
let manifest = Manifest::new(vec![Wasm::data(wasm)]).with_allowed_path("/path/on/disk", "/plugin/path");
let plugin = Plugin::new_with_manifest(&mut context, &manifest, false)?;
...