Generate code for the cluster API

[pietern]

This doesn't work yet.

When I run yarn test under packages/databricks-vscode I get:

src/cluster/ClusterModel.ts:1:41 - error TS2307: Cannot find module '@databricks/databricks-sdk' or its corresponding type declarations.

The file cluster.ts is entirely generated. For code, see https://github.com/databricks/eng-dev-ecosystem/pull/3.

@fjakobs No need to merge this until it works end-to-end. If you have time you might want to take a look at fixing downstream usage. Integration pains w.r.t. naming of these things illustrate that we should be using generated code sooner rather than later. Too much room for ambiguity and errors.