[Feature Request] add get update handle and set run ID

[cretz]

Describe the solution you'd like

Should be able to get an update handle from a workflow handle. Also since updates are always run-specific, when using "start", the update handle should use the returned run ID regardless of what user passed in (but can still leave optional when using getter). And for the update-handle getter, we should document very clearly the consequences of getting an update handle on a workflow handle with no run ID.