[Feature Request] Handle possibility of update-before-start

[cretz]

Describe the solution you'd like

See similar Python issue at temporalio/sdk-python#462.

Basically activation job events are ordered to have update first then start if they come in the same task. But what if someone dynamically registers a handler? It won't work on start. We likely need some buffer-for-life-of-task approach to updates so dynamic handlers work on start.