wasi-nn: only support components

[abrown]

This is a relatively drastic change to remove all the preview1, WITX-based support from Wasmtime's wasi-nn implementation. This has ramifications:

• Wasmtime will not be able to run core modules that target wasi-nn, either through the CLI or the embedding API
• Sightglass will not be able to benchmark wasi-nn-using code until the bench-api learns how to handle components
• the wasi-nn bindings crate is no longer usable with Wasmtime (it only understands the preview1 ABI)
• all wasi-nn testing in Wasmtime will now use components (the examples are left for a later refactor)

Why this change, then? The WebAssembly specification has pushed ahead, defining new functionality that depends on component model features (e.g., resources). So far, the wasmtime-wasi-nn has had its preview1- and preview2-ABI implementations coexisting side by side, wit.rs and witx.rs. This was only possible because the WIT and WITX definitions were roughly similar. But I plan to update the Wasmtime implementation to support the new spec changes, which means the older preview1-ABI WITX code would immediately be out of date. And the differences between the updated WIT code and the old WITX code would only grow over time.

[alexcrichton]

From a technical perspective this all looks fine to me, thanks! For a "is this the time to do this or not" I'd defer to the wasi-nn subgroup, which I know you're intimately tied into as well and likely have a consensus before making this PR

[abrown]

Superseded by #8873.