Flag for proto message to signal need to override upon and emit events with message itself (and not its fields)

[ranchalp]

In some cases it comes handy to have the upon and emit functions directly handle the entire proto message and not receive each of its fields in a separate parameter, for example:

checkpointpbdsl.UponStableCheckpoint(m,  func(sn types.SeqNr, snapshot *commonpbtypes.StateSnapshot, cert map[t.NodeID][]uint8) error {
	// need to reconstruc stableCheckpoint for the function
	chkp := &checkpoint.StableCheckpoint{
   		 Sn: sn,
   		 Snapshot: snapshot,
   		 Cert: cert,
   	 }
   	 if err := appModule.appLogic.Checkpoint(stableCheckpoint); err != nil {
   		 return nil, fmt.Errorf("error handling StableCheckpoint event: %w", err)
   	 }
   	 
   	 return nil
    })

This function would be greatly simplified if the parameter was the proto message itself (removing boilerplate code).