add total and working time to model, tool, and subtask events

[jjallaire]

All events currently carry a timestamp for the clock time when they start, with this PR we add several fields to events to track the working time at which they start as well as the duration of clock and working time consumed by events which occur over a span (ModelEvent,ToolEvent,SubtaskEvent).

Here are the fields and which events they apply to:

Event	Description	Events
timestamp	Clock time at which event occurred.	All
completed	Clock time at which event completed.	ModelEvent, ToolEvent, SubtaskEvent
working_start	Working time (within sample) when the event occurred.	All
working_time	Working time consumed by the event	ModelEvent, ToolEvent, SubtaskEvent

ForToolEvent and SubtaskEvent the completed/working tracking is trivial as we start execution, create the event, and update the event all in the same stack frame.

For ModelEvent, we start execution outside of a semaphore and then potentially several ModelEvent instances can be added to the transcript (none of which are visible in the outer stack frame). We navigate this by searching for and updating the last ModelEvent in the transcript from the outer stack frame.

@dragonstyle We need to figure out how to display this data in the viewer.

Viewer Changes

Timing In model event (all subsection)

Event titles now include working time (model, tool calls, subtask)

Working start time for all events