JupyterLab Plugin

In #54 we added an API that allows users to configure admin-defined fields when starting a cluster. This API provides an endpoint for discovering what options exist (name, type, description). In the Python client library we use this information to build a `ipywidget` interface for configuring the cluster. We could do the same here with a JupyterLab plugin.

I'm not sure what this would look like. I suspect it'd be easier to build a new plugin than modify [the existing dask plugin](https://github.com/dask/dask-labextension/), as we have features they don't need, and vice versa (e.g. we don't need any of the proxying support).