WIP WorkflowUI observation with delegate methods

[amorde]

Compare with https://github.com/square/workflow-swift/pull/209 and https://github.com/square/workflow-swift/pull/210

Pros

• Matches existing APIs for Workflow observation
• Adding new events is mostly backwards compatible (via default implementation in protocol extension)

Cons

• Verbose definition & implementation
• No way to pattern match over a concrete list of possible events
• Slightly cumbersome to port to another observation system (ex. FRP frameworks) as it would require re-defining the data in a tuple or a custom type