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/examples/wasi-nn-rust-bindings/README.md

@@ -0,0 +1,65 @@
+wasi-nn Rust Bindings
+=====================
+
+This crate contains API bindings for [wasi-nn] system calls in Rust. It is similar in purpose to the [wasi bindings] but
+this crate provides access to the optional neural network functionality from WebAssembly.
+
+[wasi-nn]: https://github.com/WebAssembly/wasi-nn
+[wasi bindings]: https://github.com/bytecodealliance/wasi
+
+> __NOTE__: These bindings are experimental (use at your own risk) and subject to upstream changes in the wasi-nn
+> specification.
+
+> __NOTE__: In the future this crate may be (should be) moved to its own repository, like the [wasi bindings].
+
+### Use
+
+Depend on this crate in your `Cargo.toml`:
+
+```toml
+[dependencies]
+wasi-nn = "0.1.0"
+```
+
+Use the wasi-nn APIs in your application:
+
+```rust
+use wasi_nn;
+
+unsafe {
+    wasi_nn::load(
+        &[&xml.into_bytes(), &weights],
+        wasi_nn::GRAPH_ENCODING_OPENVINO,
+        wasi_nn::EXECUTION_TARGET_CPU,
+    )
+    .unwrap()
+}
+```
+
+Compile the application to WebAssembly:
+
+```shell script
+cargo build --target=wasm32-wasi
+```
+
+Run the generated Wasm in a runtime supporting wasi-nn. Currently Wasmtime has experimental support using the Wasmtime
+APIs; see [main.rs](../main.rs) for an example of how this is accomplished.
+
+### Generation
+
+This crate contains code ([`src/generated.rs`](src/generated.rs)) generated by 
+[`witx-bindgen`](https://github.com/bytecodealliance/wasi/tree/main/crates/witx-bindgen).
+To regenerate this code, run `witx-bindgen` against the [`wasi-nn` WITX file](https://github.com/WebAssembly/wasi-nn/blob/master/phases/ephemeral/witx/wasi_ephemeral_nn.witx):
+
+```shell script
+.../crates/witx-bindgen$ cargo run .../wasi-nn/phases/ephemeral/witx/wasi_ephemeral_nn.witx
+```
+
+### License
+
+This project is licensed under the Apache 2.0 license. See [LICENSE](LICENSE) for more details.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, 
+as defined in the Apache-2.0 license, shall be licensed as above, without any additional terms or conditions.