Cluster `init_scripts` option no longer supported in SDK

[adampicwell]

According to the Databricks 2.0 API documentation, init_scripts are supported as an option when creating clusters (https://docs.databricks.com/dev-tools/api/latest/clusters.html). It looks like this kwarg was removed here: https://github.com/databricks/databricks-cli/pull/325 accidentally due to some code autogeneration (see this comment and response: https://github.com/databricks/databricks-cli/pull/325#discussion_r496278147).

This option should still work in the CLI as the CLI code does not use the ClusterService.create_cluster method and takes in a raw JSON payload, but does break the SDK's ability to create clusters with init_scripts. I recognize this is somewhat of a bizarre use case, and that you likely don't anticipate others using the underlying SDK, but it would be great to have this functionality back in the SDK.