wasi-nn: add named models (#6854)

* wasi-nn: add [named models]

This change adds a way to retrieve preloaded ML models (i.e., "graphs"
in wasi-nn terms) from a registry. The wasi-nn specification i

wasi-nn: add named models (#6854)

* wasi-nn: add [named models]

This change adds a way to retrieve preloaded ML models (i.e., "graphs"
in wasi-nn terms) from a registry. The wasi-nn specification includes a
new function, `load_by_name`, that can be used to access these models
more efficiently than before; previously, a user's only option was to
read/download/etc. all of the bytes of an ML model and pass them to the
`load` function.

[named models]: https://github.com/WebAssembly/wasi-nn/issues/36

In Wasmtime's implementation of wasi-nn, we call the registry that holds
the models a `GraphRegistry`. We include a simplistic `InMemoryRegistry`
for use in the Wasmtime CLI (more on this later) but the idea is that
production use will involve some more complex caching and thus a new
implementation of a registry--a `Box<dyn GraphRegistry>`--passed into
the wasi-nn context. Note that, because we now must be able to `clone` a
graph out of the registry and into the "used graphs" table, the OpenVINO
`BackendGraph` is updated to be easier to copy around.

To allow experimentation with this "preload a named model"
functionality, this change also adds a new Wasmtime CLI flag: `--graph
<encoding>:<host dir>`. Wasmtime CLI users can now preload a model from
a directory; the directory `basename` is used as the model name. Loading
models from a directory is probably not desired in Wasmtime embeddings
so it is cordoned off into a separate `BackendFromDir` extension trait.

* wasi-nn: add "named model" test

Add a new example crate which loads a model by name and performs image
classification. It uses the same MobileNet model as the existing test
but a new version of the Rust bindings. The new crate is built and run
with the new CLI flag in the `ci/run-wasi-nn-example.sh` script.

prtest:full

* review: rename `--graph` to `--wasi-nn-graph`

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