Idea: Investigate how easy it would be to store a stream from a ledger as a file (Protobuf or (preferably) JSON encoded), which can then be replayed as a BatchSource. This could be used for testing specific input from any ledger. The file could possibly be generated by running a projection into (hypothetical) file actions (with a projection that just takes the event directly and persists to file), or by some custom code that uses the gRPC client.