Send information about output node data sent to frontend

There isn't any indication that a node is an output node sent by `object_info` currently. You have to assume the node is an output if the backend sends back an output data when it's executed. I would need this for adding extra output slots to the backend nodes for use with events so I can pass the result to the UI

So at least indication that a node has `OUTPUT_NODE = True` would be nice. And better if the nodes could send back the expected shape of the output data in the `"UI"` argument also. Some nodes can send back data besides images for any purpose