[WIP] Add experiment logging support for BrainTrust

[ankrgyl]

[abrenneke]

Not strictly necessary to include in the MVP, but if it's possible to emit an event each time a node executes for each test case, we can incrementally log span progress to BrainTrust, and then you'd be able to see performance metrics across the whole execution tree.

Ah yeah I can just pipe in the events from the graph execution!