Figure out how to support GUWT worker identity with ImpostorWorkflow

[zach-klippenstein]

In square/workflow#961 (comment) I missed the fact that, even without adding keys to Workflow, we still need a way to express a worker's type through ImpostorWorkflow. We currently use KType for worker identity, but that type is not serializable, and definitely not deserializable.

We don't want to take on the major change of adding a non-snapshottable workflow type, which leaves us with the following options:

1. Write our own (de)serializer for KType. This could be hard to do right, there are a lot of edge cases. It's also a waste of effort, since we have no actual state to snapshot anyway.
2. Figure out some dirty hack for making WorkflowIdentifier accept non-serializable types and avoid snapshotting.

Since 1 is wasted work, I think 2 is better, even though i am not optimistic we'll be able to come up with a solution that isn't hacky. As long as give the hack enough friction, it is unlikely to wreak a lot of havoc.