Add an initial wasi-nn implementation for Wasmtime (#2208)

* Add an initial wasi-nn implementation for Wasmtime

This change adds a crate, `wasmtime-wasi-nn`, that uses `wiggle` to expose the current state of the wasi-nn API and `openvino` to implement the exposed functions. It includes an end-to-end test demonstrating how to do classification using wasi-nn:
 - `crates/wasi-nn/tests/classification-example` contains Rust code that is compiled to the `wasm32-wasi` target and run with a Wasmtime embedding that exposes the wasi-nn calls
 - the example uses Rust bindings for wasi-nn contained in `crates/wasi-nn/tests/wasi-nn-rust-bindings`; this crate contains code generated by `witx-bindgen` and eventually should be its own standalone crate

* Test wasi-nn as a CI step

This change adds:
 - a GitHub action for installing OpenVINO
 - a script, `ci/run-wasi-nn-example.sh`, to run the classification example

crates/wasi-nn/src/witx.rs

@@ -0,0 +1,40 @@
+//! Contains the macro-generated implementation of wasi-nn from the its witx definition file.
+use crate::ctx::WasiNnCtx;
+use crate::ctx::WasiNnError;
+
+// Generate the traits and types of wasi-nn in several Rust modules (e.g. `types`).
+wiggle::from_witx!({
+    witx: ["$WASI_ROOT/phases/ephemeral/witx/wasi_ephemeral_nn.witx"],
+    ctx: WasiNnCtx,
+    errors: { errno => WasiNnError }
+});
+
+use types::Errno;
+
+/// Wiggle generates code that performs some input validation on the arguments passed in by users of
+/// wasi-nn. Here we convert the validation error into one (or more, eventually) of the error
+/// variants defined in the witx.
+impl types::GuestErrorConversion for WasiNnCtx {
+    fn into_errno(&self, e: wiggle::GuestError) -> Errno {
+        eprintln!("Guest error: {:?}", e);
+        Errno::InvalidArgument
+    }
+}
+
+impl<'a> types::UserErrorConversion for WasiNnCtx {
+    fn errno_from_wasi_nn_error(&self, e: WasiNnError) -> Errno {
+        eprintln!("Host error: {:?}", e);
+        match e {
+            WasiNnError::OpenvinoError(_) => unimplemented!(),
+            WasiNnError::GuestError(_) => unimplemented!(),
+            WasiNnError::UsageError(_) => unimplemented!(),
+        }
+    }
+}
+
+/// Additionally, we must let Wiggle know which of our error codes represents a successful operation.
+impl wiggle::GuestErrorType for Errno {
+    fn success() -> Self {
+        Self::Success
+    }
+}