Rename Run to Tracker

[andyk]

We could go further than #335 did and instead rename pcs.Run to pcs.Tracker, and then make Component.run() and Command.run() return a Tracker.

• a Tracker is like a logger where everything that it logs is associated with a unique ID
• a Tracker maps to an MLflow Run.
• Trackers can be nested, in which case they show up as nested in the MLflow UI
• Children trackers should have links to parents for sure (they do today)
• Should parents have links to children?
• A Tracker is a Component
• Run is now only a verb in PCS/AgentOS (not a noun anymore) and corresponds to running an MLflow Project.
• In this case we would rename agentos.AgentRun to agentos.AgentTracker