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/README.md

@@ -0,0 +1,38 @@
+# wasmtime-wasi-nn
+
+This crate enables support for the [wasi-nn] API in Wasmtime. Currently it contains an implementation of [wasi-nn] using
+OpenVINO™ but in the future it could support multiple machine learning backends. Since the [wasi-nn] API is expected
+to be an optional feature of WASI, this crate is currently separate from the [wasi-common] crate. This crate is
+experimental and its API, functionality, and location could quickly change.
+
+[examples]: examples
+[openvino]: https://crates.io/crates/openvino
+[wasi-nn]: https://github.com/WebAssembly/wasi-nn
+[wasi-common]: ../wasi-common
+
+### Use
+
+Use the Wasmtime APIs to instantiate a Wasm module and link in the `WasiNn` implementation as follows:
+
+```
+let wasi_nn = WasiNn::new(&store, WasiNnCtx::new()?);
+wasi_nn.add_to_linker(&mut linker)?;
+```
+
+### Build
+
+This crate should build as usual (i.e. `cargo build`) but note that using an existing installation of OpenVINO™, rather
+than building from source, will drastically improve the build times. See the [openvino] crate for more information
+
+### Example
+
+An end-to-end example demonstrating ML classification is included in [examples]:
+ - `tests/wasi-nn-rust-bindings` contains ergonomic bindings for writing Rust code against the [wasi-nn] APIs
+ - `tests/classification-example` contains a standalone Rust project that uses the [wasi-nn] APIs and is compiled to the 
+ `wasm32-wasi` target using the `wasi-nn-rust-bindings`
+
+Run the example from the Wasmtime project directory:
+
+```
+ci/run-wasi-nn-example.sh
+```