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/classification-example/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "wasi-nn-example"
+version = "0.19.0"
+authors = ["The Wasmtime Project Developers"]
+readme = "README.md"
+edition = "2018"
+publish = false
+
+[dependencies]
+wasi-nn = { path = "../wasi-nn-rust-bindings", version = "0.1.0" }
+
+# This crate is built with the wasm32-wasi target, so it's separate
+# from the main Wasmtime build, so use this directive to exclude it
+# from the parent directory's workspace.
+[workspace]