Add method to stream text to node UI

[christian-byrne]

Add PromptServer method that can be called by nodes to send text progress to client UI like

from server import PromptServer

class Node:
    @classmethod
    def INPUT_TYPES(cls):
        return {
            "hidden" : {
                "unique_id": "UNIQUE_ID",
            }
        }

    RETURN_TYPES = (,)
    FUNCTION = "send_message"

    def send_message(self, unique_id=None):
        PromptServer.instance.send_progress_text("test message", unique_id)
        return (,)

which can be used with LLM nodes or to show messages when a node is executing/executed