Emit events for observation

[dfarr]

Implement a pub/sub inspired system that emits information from the sdk event loop. This information can be used to integrate with third party observation frameworks such as opentelemetry.

At a minimum we should emit:

• function execution started
• function attempt started
• function attempt finished (successfully or unsuccessfully)
• function execution finished (successfully or unsuccessfully)

We also need to include call stack information so subscribers can now about the nested nature of function calls.