Make it easy to update models to latest version

[robertknight]

ocrs-cli will automatically download models on first run. It would be useful if it could automatically download updated models when they become available.

Some considerations and constraints:

• There should be a way to prevent this. Some users might want the tool to never download anything, or might want to be in control of when model updates happen
• There should be a fallback to the existing models if the download fails
• There needs to be a way of displaying a human readable description of the current/available model versions. See also https://github.com/robertknight/rten/issues/20.
• There is a trade-off between binary size / compile time and flexibility to allow model updates to use new operators. ocrs as a native library currently loads models with the default operator registry which supports all operators. However the WebAssembly API reduces binary size by using a custom operator registry with only the required ops enabled. We might want to do the same thing with the native API in future too.