[batch] Create Work Item Scheduler

[pdames]

The Ray Work Item Scheduler takes work items from the batch FnApiRunner's topological scheduler as input, and submits them for execution as Ray worker tasks. Its design should be based on the FnApiRunner's WorkerHandlerManager.

Ray worker tasks should be based on Beam's EmbeddedPythonWorker and other worker handlers. Each Ray worker task takes a ProcessBundleRequest proto to execute, and stores/retrieves its state as object references in the central Pipeline State Manager.

[pdames]

This work is required as part of #2