Add support for Assembly Progress

[kvz]

Our API supports predicting the duration of many Assemblies now, as well as streaming updates around this to the client. But I don’t think our node sdk supports this yet. We should think of an elegant way to roll out support for this in the Node SDK first since that is our reference implementation. We should consider that this SDK could ultimately become our JavaScript SDK supporting many runtimes (browsers, deno, etc) and if and how that should influence the design and implementation of this feature