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/lib.rs

@@ -0,0 +1,26 @@
+mod ctx;
+mod r#impl;
+mod witx;
+
+pub use ctx::WasiNnCtx;
+
+// Defines a `struct WasiNn` with member fields and appropriate APIs for dealing with all the
+// various WASI exports.
+wasmtime_wiggle::wasmtime_integration!({
+    // The wiggle code to integrate with lives here:
+    target: witx,
+    // This must be the same witx document as used above:
+    witx: ["$WASI_ROOT/phases/ephemeral/witx/wasi_ephemeral_nn.witx"],
+    // This must be the same ctx type as used for the target:
+    ctx: WasiNnCtx,
+    // This macro will emit a struct to represent the instance, with this name and docs:
+    modules: {
+        wasi_ephemeral_nn => {
+          name: WasiNn,
+          docs: "An instantiated instance of the wasi-nn exports.",
+          function_override: {}
+        }
+    },
+    // Error to return when caller module is missing memory export:
+    missing_memory: { witx::types::Errno::MissingMemory },
+});