Add command to cli to dump json views of the models.

[Baccata]

The smithy-model library is awesome, but is JVM-only. As a low-cost way of building more cross-platform tooling (in particular dynamic tooling), we could parse models from json into a custom, pure-scala representation. That representation would obviously not have as much capabilities as the pure java one, but it could be loaded into scala-js/scala-native programs.

In any case, we'll need an easy way to dump the json view of models, and an additional command to the CLI would be great for that.