WIP UI Observation with typed events

[amorde]

Compare with https://github.com/square/workflow-swift/pull/208 and https://github.com/square/workflow-swift/pull/210

Pros

• More concise observer protocol definition
• Easily portable to other observation systems (ex. FRP frameworks) by passing the types through
• Can easily be extended / customized to support listening to only a single event type
• Adding fields to an existing event is backwards compatible
• Adding new events is backwards compatible

Cons

• No way to pattern match over a concrete list of possible events
• Adding a new case to the enum is backwards incompatible
• Does not match existing observation APIs for Workflow