rust: Workflow builder api

[emesare]

Specifically want this for the rust api.

Right now workflows are in two states: registered (immutable) and unregistered (mutable).

The distinction itself is hidden in the type system making it very easy for a user to mistake an API meant for unregistered workflows for one they can use on a registered workflow.

Adding a WorkflowBuilder or something that acts as a mutable workflow would allow us to use the type system to enforce correct behavior.